Fun Fact: Eleven is the only number this sneaky trick works on

If the difference between the sum of a number’s odd and even digits is divisible by 11, so is the number.

For example, 6,050 has digits 6+5 = 11 (odd positions) and 0+0 = 0 (even positions). The difference is 11, which is divisible by 11—so the whole number is too! This special rule works only with 11 because of the way powers of 10 alternate mod 11.