You can also

Multiply one side, divide the other to get the base number to get the base number to 100.

6% of 50 == 3% of 100 == 3

5% of 20 == 1% of 100 == 1

7% of 50 == 3.5℅ of 100 == 3.5

14% of 200 == 7% of 100 == 7

Edit* as pointed out below, this one was incorrect.

14% of 200 == 28% of 100 == 28

You don't have to stop at percentage either.

25 * 16 = 100 * 4

If you're doing addition, you can add and subtract.

27 + 13 = 20 + 20

Most of the difficulty is in realizing that one number or the other can be brought up to a nice round number, making the equation simple to do in your head. And obviously, just getting to around number on one side doesn't always make it easy on the other side.

I always loved screwing with math problems to make them easier, which is weird, because overall, I don't really care for math.

I also do shit like borrowing a couple of numbers to make the equation easy and then pull them back out.

392 / 4 == (400 - 8) / 4 == (100-2) = 98

376 / 4 == (400-24)/4 =100-6=94

Of course it goes up a level when the remainder isn't evenly divisible. But I still find it's something I can handle in my head.

371 / 4 == (400-29)/4= 100-7¼ =100-7.25=92.75