I hate the imperial measurement system, and can confidently say that anyone who doesn't recognize the superiority of metric is a freaking idiot. How nice it must be for you Aussies, Germans and Koreans to drill an 8mm hole, realize you need it a smidgen bigger, and yell down the ladder for a 9mm bit. Versus us Yankees drilling a 7/32 hole, then having to do an equation in your head to calculate if you need a 3/16 or a 1/4.
For the non-mathematically-gifted like me, dividing things with fractions is the worst. For example, when doing DIY projects you often have to calculate the midpoint of a particular piece—whether it's wood, fabric or metal—and I'd be constantly scrawling equations onto the piece of wood I was working and having to sand the marks off afterwards. That is, until I learned this simple tip to easily find the exact midpoint without having to divide fractions.
Let's say I want to find the midpoint of the board above. We take a tape measure to it...
...and see it's 17-something. That's all you're looking for, ignore the finer gradations.
Then we take note of the nearest even number, whether higher or lower than the actual measurement. In this case the nearest even number is 18.
In other words, if the measurement had been 16-something (simulated below) we'd round down to 16.
Then you take that even number and divide it in two. Even a relatively thick-headed person like myself can quickly calculate that half of 18 is 9.
So we draw a line at the 9-inch mark on the board.
Then we flip the tape measure around...
...and mark 9 inches going the other way.
Now we have two marks that are less than an inch apart.
If you have managed to graduate with a design degree, or if you build things for a living, or have any visual acuity whatsoever, it's super-easy to eyeball the exact midpoint of two lines less than an inch apart.
Mark it, and that's it, you're done.
If you want to double-check, just run the tape measure, no math necessary. Here we see we've got 8 5/8.
Then flip the tape measure, and confirm we've got 8 5/8 going the other way.
The best part of this system is that it's foolproof. In other words, if I was dumb enough that I divided 18 by 2 and came up with 8, and marked 8 inches from each side, the two marks would be a lot wider than 1 inch apart, and I'd know I screwed up and would recalculate before cutting anything.
I can't remember which magazine, website or blog I first saw this tip on—it was years ago—so I cannot give credit where it's due. Nevertheless, I'm sure this is an old and fairly common carpenter's tip.