Speedy math in your head

I like this bit of trickery, from Wired's How-to Wiki:

Do Speedy Math in Your Head

Illustration by Lab Partners

Arthur Benjamin is a wizard at math. Literally. At Hollywood's Magic Castle, the world-famous conjurer's club, he wows the crowd by multiplying big numbers — quick, what's 57,682 squared? — faster than you can use a calculator. Here he shares three cool tricks. (The answer, by the way: 3,327,213,124. See how easy it is?)

Square off

To square a number like 14, identify the closest round number — in this case, 10. Since you subtracted 4 to get 10, add 4 to 14 to get 18 and multiply that by 10. Add to that the square of 4: 180 + 16 = 196.

11 times any two-digiter

To multiply, say, 11 x 32, add the digits of 32 (3 + 2 = 5) and insert the sum between them: 352. Numbers with two-digit sums use a slight variation: For 11 x 84 (8 + 4 = 12), add the 1 from 12 to the 8 and leave the 2 in the middle: 924.

Magic number

Ask a fan to think of any number. Then have them double it, add 12, divide by 2, and subtract the original number. Before they're done, tell them the answer: 6. It will always be 6.