For 11 there is an easier trick (or at least it's easier in that you can perform it in your head faster). Starting from the left add every other digit => n1, S

For 11 there is an easier trick (or at least it's easier in that you can perform it in your head faster). Starting from the left add every other digit => n1, Starting with the second digit add every other digit => n2 if n1 - n2 == 0 or if n1 - n2 is divisible by 11 then the number is divisible by 11. Example 13574 1 + 5 + 4 = 10 3 + 7 = 10 Since 10 - 10 == 0, the number is divisible by 11. Even cooler example 155357548018579643 1 + 5 + 5 + 5 + 8 + 1 + 5 + 9 + 4 = 43 5 + 3 + 7 + 4 + 0 + 8 + 7 + 6 + 3 = 43 43 - 43 == 0, also divisible by 11