# Old trick to add fractions



## kweinert (Apr 12, 2016)

http://www.woodworkersjournal.com/an-easy-way-to-tackle-fractions/

_"Here’s an old carpenter’s trick for solving fractions that works either with two tape measures or steel rules. If you need to add or subtract two fractional measurements, place the two side by side and align them according to the lengths that need to be added or subtracted. It doesn’t matter if the denominators of the fractions match, because you’re simply comparing two distances, not computing two fractions. For instance, in the photo here, 3-1⁄16″ (top rule) + 1-5⁄8″ (bottom rule) = 4-11⁄16″. The ends of their overlaps tell you the sum. Subtract the same way: the end of the overlap minus one rule’s length shows you the answer. No complicated math required.


-Charles Mak
Calgary, Alberta"_







Kind of a nifty idea that I hadn't thought of before.

Reactions: Great Post 1 | Way Cool 1 | Informative 1 | Creative 1 | Useful 3


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## ripjack13 (Apr 12, 2016)

nice...I like that one....


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## Kevin (Apr 12, 2016)

I can always remember the addition trick, but I can never remember the subtraction rule. I'm not good with algebra or trig but I can add and subtract fractions in my head quicker than finding two rules anyway. 

If you ask me to subtract 7 21/64 from 20 11/32 I'd have to ponder that a spell but I would still be able to do it in my head eventually. However subtracting 1/16's, 1/8's, or 1/4's from themselves and each other in the head is pretty easy.


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## DaveHawk (Apr 12, 2016)

For someone like me who is terrible at math, this is great. Thanks.


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## HomeBody (Apr 13, 2016)

Here's a fraction tip. If you want to find half of a fraction, just double the bottom number. half of 1/4 is 1/8. Gary

Reactions: Useful 1


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## jmurray (Apr 13, 2016)

Pap showed me that one years ago!


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## kweinert (Apr 13, 2016)

Kevin said:


> I can always remember the addition trick, but I can never remember the subtraction rule. I'm not good with algebra or trig but I can add and subtract fractions in my head quicker than finding two rules anyway.
> 
> If you ask me to subtract 7 21/64 from 20 11/32 I'd have to ponder that a spell but I would still be able to do it in my head eventually. However subtracting 1/16's, 1/8's, or 1/4's from themselves and each other in the head is pretty easy.




The subtraction part would work like this:

Take the original example, but you have 4 11/16 and you want to subtract 1 5/8. Put the end of one rule on the 4 11/16 mark of a second, then go back 1 5/8 on the first rule. Where it matches the second rule is the answer: 3 1/16.

Of course, using this physical method would mean that you'd need one of the rulers to be an 18" one just to be sure you could always do the subtraction.


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## JR Parks (Apr 13, 2016)

And from the tech section- Carpenter's Helper Lite a smart phone app (Lite means its a free app or you can buy the more better one) will do fractions very easy. I use to check all of my pencil scratchings on my wood.

Reactions: Informative 1


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## Mauser (Apr 14, 2016)

I use to work at a company where the draftsman would give cultists down to the 64th of an inch. It was such a pain in the ass having to show guys I was training how to convert it to 16th.


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