Vinnige wiskunde

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.


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.


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.


- from Wired