If someone ever told you that the answer to the ultimate question of life, the universe and everything is "42", tell them that is wrong. The answer is "6174" and here's why (and prepare to get your mind blown):

Take any number with 4 non-repeating digits. Say 1562.

Step 1: Arrange the number in ascending and then descending order
Step 2: Subtract the smaller number from the bigger number

6521 - 1256 = 5265

Repeat the steps:

6552 - 2556 = 3996
9963 - 3699 = 6264
6642 - 2466 = 4176
7641 - 1467 = 6174

Try any 4-digit number with non-repeating digits, and you'll *always* get 6174.

Pretty cool, huh?

6174 is known as Kaprekar's constant. The math operation above, discovered by Indian mathematician D.R. Kaprekar, will reach 6174 after at most 7 steps (if you did more than 7 iterations, check your arithmetics).

See also: Math T-shirts at the NeatoShop