Divisibility by 3 depends on whether which quantity is a multiple of 3?

1. Add up every digit of the number. 2. The number is divisible by 3 if and only if that sum is divisible by 3 (the same works for 9). 3. The last digit or last two digits are used for 2, 4 and 5, not for 3. _Source: Wikipedia, Divisibility rule, section Divisibility by 3 or 9_