Question

Which is not a prime number 19,29,39

Answer 1

The composite # is 39

Answer 2

Prime numbers are numbers that are only divisible by 1 and themselves. So, to tell if a number is prime, look at its factors.
19:
1 x 19
29:
1 x 29
39:
1 x 39
3 x 13
So, because 39 has more than one set of factors, it's NOT a prime number.