Question

of the following numbers , which one os prime : 14,13,21,23

Answer 1

Prime refers to numbers that cannot be divided by any number but themselves and 1. 

14 can be eliminated, as it is even (no primes except 2 are even) 
13 has no divisors. It is prime. 
21 has 4 divisors. It is not prime. 
23 has no divisors. It is also prime.

Two numbers of this list are prime; 13 and 23.