[Math is Easy, for real]

September 19th, 2007

Math is Easy, for real

I decided that since I know a few neat math tricks and since I happen to run across a few of other people’s math tricks, I should compile them. So, skipping the boring ones that everyone already knows, here you go.

Multiplication

x is the number to multiply

Multiply by 5: 10x / 2
Multiply by 6: 3x * 2
Multiply by 9: 10x - x
Multiply by 12: 10x + 2x
Multiply by 13: 3x + 10x
Multiply by 14: 7x * 2
Multiply by 15: 10x + 5x
Multiply by 16: 2(2(2(2x)))
Multiply by 17: 7x + 10x
Multiply by 18: 20x - 2x
Multiply by 19: 20x - x
Multiply by 24: 3(8x)
Multiply by 27: 30x - 3x
Multiply by 45: 50x - 5x
Multiply by 90: 9x * 10
Multiply by 98: 100x - 2x
Multiply by 99: 100x - x

Percentages

Trying to figure out what 13% of 542 is? Shouldn’t be all that hard, here’s what I do.
10% of 542 = 54.2
1% of 542 = 5.42
5.42 * 3 = 16.26
16.26 + 54.2 = 70.46

In everyday situations that might be difficult, but you can usually just ignore the fraction portion and get a rough estimate quickly. So, to go over as an explanation. Figuring out 10% is easy, and figuring out 1% is easy. So just multiply them accordingly.
24% = (10% * 2) + (1% * 4)

If anyone has any tricks they enjoy, throw ‘em in comments.

Posted in: General » by: Smack

[Math is Easy, for real]