Is 19 a multiple of 3?

Answer 1

No.

To check if a number is a multiple of 3 add its digits together and if this sum is a multiple of 3, then so is the original number. This sum can be continued until a single digit remains; if the single digit is 3, 6 or 9 then the original number is a multiple of 3, otherwise not.

19 → 1 + 9 = 10

10 → 1 + 0 = 1 which is not 3, 6 or 9, so 19 is not a multiple of 3.