there’s a whole series of quick checks for divisibility:
- number ends in even digit = divisible by 2
- number ends in 0 or 5 = divisible by 5
- digits add up to multiple of 3 = divisible by 3 (ex. 54 → 5+4 = 9)
- digits add up to 9 = divisible by 9
- (from 11 to 99) doubled digits = divisible by 11
ex. 66
- doubled digits = divisible by 11
- ends in even digit = divisible by 2
- 6+6=12, 1+2=3 = divisible by 3
- and then working backwards, 66 is also divisible by 6, 22, and 33