How do you know if the number is divisible by 15?

All numbers divisible by 5 (of which 15 is a multiple) have a final digit of 0 or 5.

All numbers divisible by 3 (of which 15 is a multiple) have the sum of the digits totalling 3 or a multiple of 3.

Therefore, a number is divisible by 15 if the sum of its digits total 3 or a multiple of 3 and its final digit is 0 or 5.

Example : 32085 ; 3 + 2 + 0 + 8 + 5 = 18 which is divisible by 3. Final digit 5.

This number is divisible by 15. (32085 รท 15 = 2139)

7420 : 7 + 4 + 2 + 0 = 13. This number is not divisible by 15.