# do simple math in your head



## woodnthings (Jan 24, 2009)

I've seen the infomercial Brainetics, with the math wizard and his kids, pretty cool. I don't know how they do it, but here's what I do:

example: what's 16 X 24?

10 X 24 is 240, 5 X 24 is half of 240 or 120. Add them. need 1 more 24 to make 16, 10 +5 + 1 = 16

240 + 120 = 360 + 24 = 384

I use 10's and 5's and and then single numbers under 5. Or 5 minus 1 is 4. 

I'm lost on division.... :thumbdown:

Anybody got other tips or tricks?


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## Dominick (May 2, 2011)




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## WillemJM (Aug 18, 2011)

woodnthings said:


> I've seen the infomercial Brainetics, with the math wizard and his kids, pretty cool. I don't know how they do it, but here's what I do:
> 
> example: what's 16 X 24?
> 
> ...


Bill, I'm not smart enough to do that. Found that out the hard way. The last time I tried that I cut my board too short and had throw it away.

So now I do it on five different calculators and if I get the same answer five times, I then do it on my laptop. If I still get the same answer, I measure six times on my board, using a different color pencil each time. If all the marks align, I cut my board. So, now I am trying to figure out why I still cut my last board too short???? Any suggestions. :blink:

Just joking.


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## Kenbo (Sep 16, 2008)

You do it the same way that I do Bill. It pretty much works for me and when it doesn't, I use Dominick's method.


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## cabinetman (Jul 5, 2007)

Now this is an interesting topic. There was a time immediately after hurricane Wilma when in my area we lost all power for about 17 days. There was really nothing to do. It was extremely quiet outside, as there was little to no automobile traffic because there was no gas. The roads were strewn with tree limbs and debris and many were impassable. So, when I say, there was nothing to do, you didn't want to do anything. The daytime temperatures got up to the mid to high 90's. You could stand still and just sweat.

I found at that time what a perfect time to practice doing math in my head. I could just sit outside under the shade of what was left of a huge Avocado tree and practice additions and subtractions. My methods were a lot like yours. I used a simple step by step evaluation that fell in place.

I will say if it gets too difficult you could use the binary system and then you'll only have zeros and ones to deal with. But, if you really want to stimulate your mind, use the octal, or hexadecimal system. Like I said, that there wasn't anything to do, and if I made a mistake, it didn't matter a hill of beans. I could just erase my mental image and start over. A wrong answer didn't create any disasters.

But, if I'm figuring out a woodworking or cabinet layout, a wrong answer could wind up with a kitchen countertop that's 6' high, and wall cabinets 6" high, and instead of fitting on a wall that's 7' long they end up 17' long. So, what was the consequence of making a mistake. Lets see...lost time, material waste, and aggravation. If it's necessary to do any math, I write everything down so I have a record of what the details are and what they apply to. I don't do any project planning and proceed without making a drawing of some sort. 

It does seem very simple to start a project with what's in our mind. But having some reference to back up our thoughts might be easier than trying to remember all the numbers. As for your despair with division, *try this method*.



















.


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## woodnthings (Jan 24, 2009)

*cell and "smart" phones have a calculator*

No need to carry a separate calculator anymore if you're packin' smart phone.
Sometimes while driving I need to calculate something, hands will remain in the 2 and 10 steering position, so I try to do it in my head. It impresses my son, but he won't let on, he just reaches for his phone. :laughing:

I'd like to get the Brainetics program someday. It's very impressive to me anyway.:yes:


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## cabinetman (Jul 5, 2007)

woodnthings said:


> Sometimes while driving I need to calculate something, hands will remain in the 2 and 10 steering position, so I try to do it in my head. It impresses my son, but he won't let on, he just reaches for his phone.


What might impress your son more, and act as a role model for him would be to concentrate on your driving.:yes:




















.


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## woodnthings (Jan 24, 2009)

*he's been properly trained, thank you*



cabinetman said:


> What might impress your son more, and act as a role model for him would be to concentrate on your driving.:yes:
> 
> 
> 
> ...


I've taken 2 Police/Civilian Motorcycle Training classes, have experience on the GM proving Ground Facility, drive on icy hilly, roads all winter. He passed his cycle training exam with 100% score and drives as well or better then I. He started steering when he was 4 years old sitting on my lap. All he needs is experience and that will come with experience. 
I really don't need any advice on how to raise my son. :no:
He just received the Dean's Award in Auto Technology and will graduate at the top of his class.... no reflection on my parenting skills of course .... :yes:


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## GeorgeC (Jul 30, 2008)

woodnthings said:


> I've seen the infomercial Brainetics, with the math wizard and his kids, pretty cool. I don't know how they do it, but here's what I do:
> 
> example: what's 16 X 24?
> 
> ...


That is pretty much how I do it. I break the problem into smaller parts. Sometimes I may vary it a bit.

Division is pretty much the same thing. Break the problem into smaller parts.

George


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## cabinetman (Jul 5, 2007)

woodnthings said:


> I've taken 2 Police/Civilian Motorcycle Training classes, have experience on the GM proving Ground Facility, drive on icy hilly, roads all winter. He passed his cycle training exam with 100% score and drives as well or better then I. He started steering when he was 4 years old sitting on my lap. All he needs is experience and that will come with experience.
> I really don't need any advice on how to raise my son. :no:


What's all that have to do with some well intended safety comments?



















.


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## del schisler (Nov 5, 2009)

WillemJM said:


> Bill, I'm not smart enough to do that. Found that out the hard way. The last time I tried that I cut my board too short and had throw it away.
> 
> So now I do it on five different calculators and if I get the same answer five times, I then do it on my laptop. If I still get the same answer, I measure six times on my board, using a different color pencil each time. If all the marks align, I cut my board. So, now I am trying to figure out why I still cut my last board too short???? Any suggestions. :blink:
> 
> Just joking.


cut from the wrong end ??


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## woodnthings (Jan 24, 2009)

*nothing of course*



cabinetman said:


> What's all that have to do with some well intended safety comments?
> 
> 
> 
> ...


When I want your parenting advice I'll ask for it... just sayin' :smile:


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## cabinetman (Jul 5, 2007)

woodnthings said:


> When I want your advice I'll ask for it... just sayin' :smile:


I'm here for ya buddy...just sayin'.:yes:



















.


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## TomC (Oct 27, 2008)

woodnthings said:


> I've seen the infomercial Brainetics, with the math wizard and his kids, pretty cool. I don't know how they do it, but here's what I do:
> 
> example: what's 16 X 24?
> 
> ...


I do it basicly the same way.

I would do it this way

20 x 24 = 480

4 x 24= 96

480 - 100= 380 + 4=384


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## jschaben (Apr 1, 2010)

woodnthings said:


> I've seen the infomercial Brainetics, with the math wizard and his kids, pretty cool. I don't know how they do it, but here's what I do:
> 
> example: what's 16 X 24?
> 
> ...


Hi Bill, I do similar. In your example 16x20 = 320 and 16x4 = 64 so 320 + 64 = 384. Just cut out the 5's step. Gets a little more complicated doing 3 digit numbers, usually go with powers of ten there. Powers of ten also work with division as instead of adding the accumulated exponents, you subtract them. :smile:


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## mengtian (Nov 8, 2012)

I am a math nut. My first degree before going in the army was a B.S Math.

I still read about mathematicians and have always been fascinated with some of the greats. I like to "show off" by telling people what day of the week it would be for a given date. I have it down to about 10 seconds on good days. BTW: it is called the Doomsday Rule. Devised by Jeff Conway.

http://people.cst.cmich.edu/graha1sw/pub/doomsday/Doomsday.html

Math really helps with long range shooting which is a hobby of mine.


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## cabinetman (Jul 5, 2007)

mengtian said:


> I am a math nut. My first degree before going in the army was a B.S Math.
> 
> I still read about mathematicians and have always been fascinated with some of the greats. I like to "show off" by telling people what day of the week it would be for a given date. I have it down to about 10 seconds on good days. BTW: it is called the Doomsday Rule. Devised by Jeff Conway.
> 
> ...


That is very cool indeed.



















.


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## Itchytoe (Dec 17, 2011)

Nevermind, sending a pm instead.


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## johnnie52 (Feb 16, 2009)

Math was never one of my better subjects even when I quit driving and began designing machinery. Therefore I generally use the "HURRAY IT FITS!" method. :laughing: After that there is always cubits and spans to fall back on. :laughing::laughing:


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## GeorgeC (Jul 30, 2008)

johnnie52 said:


> Math was never one of my better subjects even when I quit driving and began designing machinery. Therefore I generally use the "HURRAY IT FITS!" method. :laughing: After that there is always cubits and spans to fall back on. :laughing::laughing:


????????

George


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## Woodwart (Dec 11, 2012)

Multiplying a 2 digit number by 11, split the digits, add them together and put the result between the two digits. So, 11 x 12 = 1 & 2, 1+2=3; 132! With larget numbers, say, 11 x 39 it still works. 3 & 9, and 3+9=12 so it gets a little trickier. Add the tens digit of the addition result to the tens digit of the multiplicand -- 30 + 12 = 42, and 9 gives you 429. I use your method for general two digit multiplying, and also when I have to multiply by 8 or 9, I multiply by 10 and then subtract one or two times the number being multiplied. During my poor period, I had to be good at mental arithmetic, because if I only had $40, I would have to add the prices of everything I threw into the cart so I wouldn't have to ask the cashier to take something back.


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## Brian T. (Dec 19, 2012)

Another useful math skill is to be able to estimate an approximate answer. Make a keystroke error on a calculator and I might miss that. Lesson: Does the calculator display a "reasonable" answer?

Example: What's the product of 17 x 24?
Minimum is more than 15x24 (that's 1.5 x 24 = 36 and add a zero) = 360
Maximum has to be less than 20 x 24 = 480

I need 35 sticks, 24" each, that's 70'. Wood is 8'. 8x8'=64' , so I need 9 sticks.


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## Smith Brother (Dec 9, 2012)

Just go to Harbor Freight, and buy a board stretcher, works for me...., I believe their price was, a dollar two ninety eight, then used my 20% coupon. final came to a dollar two eighteen. 

Great tool place, most of the time. 

Dale in Indy


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## TomC (Oct 27, 2008)

North Carolina is in the process of passing a law requiring that school must teach the multiplication tables! DUH,
Tom


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## Al B Thayer (Dec 10, 2011)

I make more mistakes woodworking trying to do the math and cut the piece. I have found many ways to do the work without doing the math. I'd rather lay the piece in place and mark and cut. I never measure and try to divide to find center. I can find the center without the math. I round everything I can to within an eighth. Anything to keep from making a math mistake.

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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## durdyolman (Mar 22, 2013)

*Here's a little mind game for yas*

Count to 1 by 1/64ths. 1/64, 1/32, 3/64, 1/16, 5/64, 3/32, 7/64, 1/8...............47/64, 3/4, 49/64, 25/32, 51/64, 13/16, 53/64.... I'm still trying to get it done in less than 2 minutes.
I'm familiar with the Pythagorum Thereom and inverse trigonometric functions, but counting is more fun.


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## Al B Thayer (Dec 10, 2011)

durdyolman said:


> Count to 1 by 1/64ths. 1/64, 1/32, 3/64, 1/16, 5/64, 3/32, 7/64, 1/8...............47/64, 3/4, 49/64, 25/32, 51/64, 13/16, 53/64.... I'm still trying to get it done in less than 2 minutes.
> I'm familiar with the Pythagorum Thereom and inverse trigonometric functions, but counting is more fun.


I can't get to 1/8. 

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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## johnnie52 (Feb 16, 2009)

Quote:
Originally Posted by *johnnie52*  
_Math was never one of my better subjects even when I quit driving and began designing machinery. Therefore I generally use the "HURRAY IT FITS!" method. :laughing: After that there is always cubits and spans to fall back on. :laughing::laughing:_

????????

George


A cubit is the distance from your elbow to your finger tips, and a span is the distance across your outstretched hand from the tip of your thumb to the tip of your pinky. HURRAY IT FITS needs no explanation. :laughing:


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## Itchytoe (Dec 17, 2011)

durdyolman said:


> Count to 1 by 1/64ths. 1/64, 1/32, 3/64, 1/16, 5/64, 3/32, 7/64, 1/8...............47/64, 3/4, 49/64, 25/32, 51/64, 13/16, 53/64.... I'm still trying to get it done in less than 2 minutes.
> I'm familiar with the Pythagorum Thereom and inverse trigonometric functions, but counting is more fun.


1/64, 2/64, 3/64, 4/64, 5/64, 6/64, ...60/64, 61/64, 62/64, 63/64, 1.

lol, that's easy as pie.


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## durdyolman (Mar 22, 2013)

Itchytoe said:


> 1/64, 2/64, 3/64, 4/64, 5/64, 6/64, ...60/64, 61/64, 62/64, 63/64, 1.
> 
> lol, that's easy as pie.


Itchy, you're my Hero! I'm always looking for an easier way to do things.:notworthy::notworthy::clap::smartass:


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## hands made for wood (Nov 2, 2007)

I found an app a few months ago on my android phone, that has every math trick in the book! I'm telling ya, my future kids will not be learning math out of the text book... they'll be learning every one of these tricks to solve mental math. Not to brag or nothing..... but they will be the smartest children in the world. hahaha

Anyways, here's the link! https://play.google.com/store/apps/...t=W251bGwsMSwyLDEsImNvbS5mdW5tYXRodHJpY2tzIl0.

This was the paid version, which I think I paid a buck for. They do have a free version as well. 

To all woodworkers this is a must have app. You will hardly believe these tricks work because they're so simple... I only wish my mom taught me all these tricks growing up!

Hope this is a help to you guys! 
Levi


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## Chaincarver Steve (Jul 30, 2011)

My wife is an expert of the day-of-the-week thing, which always impresses me. She's great with math in general. Me? Not so much. But I'm still very highly intrigued with the magic of mathematics and how our plays out in the world and nature. I love the YouTube channel "Numberphile" and it's afilliates. And I've read several books on mathematics. One of my favorites is "Is God a Mathematician?" Great stuff. I sure have a lot of interest for some who sucks terribly at math.


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## Chaincarver Steve (Jul 30, 2011)

Levi, I just downloaded the free, "lite" version. Thank you.


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## Al B Thayer (Dec 10, 2011)

johnnie52 said:


> Quote:
> Originally Posted by johnnie52 http://www.woodworkingtalk.com/f27/do-simple-math-your-head-52103/#post483348
> Math was never one of my better subjects even when I quit driving and began designing machinery. Therefore I generally use the "HURRAY IT FITS!" method. :laughing: After that there is always cubits and spans to fall back on. :laughing::laughing:
> 
> ...


Well don't leave out the rod, cable and furlong.

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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## Al B Thayer (Dec 10, 2011)

hands made for wood said:


> I found an app a few months ago on my android phone, that has every math trick in the book! I'm telling ya, my future kids will not be learning math out of the text book... they'll be learning every one of these tricks to solve mental math. Not to brag or nothing..... but they will be the smartest children in the world. hahaha
> 
> Anyways, here's the link! https://play.google.com/store/apps/details?id=com.funmathtricks&feature=search_result#?t=W251bGwsMSwyLDEsImNvbS5mdW5tYXRodHJpY2tzIl0.
> 
> ...


Take any board you can hold on top of your two outstretched index fingers. Move your fingers together sliding under the board until they touch each other. Both will always end in the center of the board. Doesn't matter where each finger starts. Always ends up in the center. This is my fast way to find center when you just want to halve something. 

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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## 4DThinker (Mar 13, 2013)

In the shop I measure by the marks on my tape. 28 and 1/2 and one 16th and a little bit instead of 28.564. Half would be 14 and 1/4 and 1/32 and an even smaller bit. Never start with the smallest mark on your tape. You can almost always half accurately between small marks on the tap simply by making a mark right between them. If you are within 1/64th of an inch you are plenty close enough in woodworking.


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## Chaincarver Steve (Jul 30, 2011)

Al B Thayer said:


> Well don't leave out the rod, cable and furlong.
> 
> Al
> 
> Friends don't let friends use stamped metal tools sold at clothing stores.


Don't forget the most precise measuring unit known to mankind: the proverbial c%nt hair.


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## MORRIS76 (May 6, 2013)

I watched a guy put up some crown moulding and he used two strips of wood to measure the span and his crown fit perfectly every time.
When I was young I liked to work on the relationship between numbers in my head. I accidentally stumbled on the "difference between two squares" and I was amazed at how it worked out the same no matter what numbers I used. Years later I had a big grin on my face when the teacher was explaining "my" discovery.


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## jjboozel (Mar 11, 2013)

Chaincarver Steve said:


> Levi, I just downloaded the free, "lite" version. Thank you.


Is there an app for iPhone?


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## TomC (Oct 27, 2008)

Chaincarver Steve said:


> Don't forget the most precise measuring unit known to mankind: the proverbial c%nt hair.


You left out "red"!
Tom


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## Al B Thayer (Dec 10, 2011)

Chaincarver Steve said:


> Don't forget the most precise measuring unit known to mankind: the proverbial c%nt hair.


That's so funny. How could we forget.

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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## Al B Thayer (Dec 10, 2011)

4DThinker said:


> In the shop I measure by the marks on my tape. 28 and 1/2 and one 16th and a little bit instead of 28.564. Half would be 14 and 1/4 and 1/32 and an even smaller bit. Never start with the smallest mark on your tape. You can almost always half accurately between small marks on the tap simply by making a mark right between them. If you are within 1/64th of an inch you are plenty close enough in woodworking.


I do the same thing. Some times half is, measure from one end and measure from the other end the same amount then divided the difference.

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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## Chaincarver Steve (Jul 30, 2011)

jjboozel said:


> Is there an app for iPhone?


Umm... I have no idea. Sorry. Mine is an Android.


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## jigs-n-fixtures (Apr 28, 2012)

jjboozel said:


> Is there an app for iPhone?


Try "mathemagics"


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## Smith Brother (Dec 9, 2012)

I use two sicks and a hand clamp to measure inside measurements, have for years. I call them CHEATER STICKS.

Then I measure the length of the sticks combined. 

Dale in Indy


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## r22yu (Apr 6, 2013)

I do something similar by breaking things into divisors of 10, 100, 1000, etc. So for 16x24 i would round it to 16x25 since 4x25=100. Then the math is either 16/4x100 = 400. Or 16 x 25 = 400. Depends which way is easier for you. After that subtract the 16 from 400.

Since all my schooling and work has been in metric, when it comes to fractions of an inch i break out the tape measure and count


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## woodnthings (Jan 24, 2009)

*I my opinion this is a great thread!*

Lot of cool posts and different ways folks get to the same result. :thumbsup:
Thanks to all for your input, and keep posting. :yes:


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## hands made for wood (Nov 2, 2007)

jjboozel said:


> Is there an app for iPhone?


Sadly I'm no help on that, however, I would be shocked if there wasn't another app just like it! Just search "math tricks" should find you an app or two! Hope this helps.


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## r22yu (Apr 6, 2013)

If you have scrap wood/paper and can't do it by head, you can scratch out some lines and do it this way.

http://lifehacker.com/5975917/quickly-multiply-big-numbers-the-japanese-way


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## Al B Thayer (Dec 10, 2011)

r22yu said:


> If you have scrap wood/paper and can't do it by head, you can scratch out some lines and do it this way.
> 
> http://lifehacker.com/5975917/quickly-multiply-big-numbers-the-japanese-way


Okay! This is something I can use. Not sure it's faster than our way. But it looks fantastic.

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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## Scluff (Jun 15, 2013)

*When I was in Highs School*

When I was in High school the way of the future was in the metric system, of course soon after graduation the metric system went away. I was pretty good at binary too lot of help that was. I do most of my cabinet building in metric; you are working with 1's, 10's, 100's, 1000's and so on. Once you start working with it you will find it gets easier. I also like math and have gone back to school at age 52 to learn more of it along c++. It is a bad habit to calculate in your head; you can always cut more off but it is hard to stretch. Plus you can really kill your margins miscalculating exotic pieces where caulk cannot be used MacGyver the job. Good luck...

cm x 0.39 = in​ in x 2.54 = cm​


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## henrylarry6 (Nov 3, 2012)

It seems difficult to me… so m not trying even to solve it out.


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## Al B Thayer (Dec 10, 2011)

Scluff said:


> When I was in High school the way of the future was in the metric system, of course soon after graduation the metric system went away. I was pretty good at binary too lot of help that was. I do most of my cabinet building in metric; you are working with 1's, 10's, 100's, 1000's and so on. Once you start working with it you will find it gets easier. I also like math and have gone back to school at age 52 to learn more of it along c++. It is a bad habit to calculate in your head; you can always cut more off but it is hard to stretch. Plus you can really kill your margins miscalculating exotic pieces where caulk cannot be used MacGyver the job. Good luck...
> 
> cm x 0.39 = in
> in x 2.54 = cm


Forget the metric. It really messes up a football field. 

Al

Friends don't let friends use stamped metal tools sold at clothing stores.


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