Did you know that any integer with digits that add up to a number divisible by 3 is itself divisible by 3.

Give it a try, type any mess of numbers and tack on the last number to make the digits sum to a number divisible by 3.

For example:

42189012

4+2+1+8+9+0+1+2 = 27 (divisible by 3).

42189012 / 3 = 14 063 004 An integer!

How does this work?