111 2151 which one is a prime number

Neither of those numbers is a prime number, as both are divisible by 3. It's easy to tell when a number is divisible by three, because all you have to do is add up all the individual numbers that make up the big number, and if THAT number is divisible by 3, then so is the big number.

1 + 1 + 1 = 3

3 is obviously divisible by 3, so no further steps need to be taken here.

2 + 1 + 5 + 1 = 9

9 is also divisible by 3.

Neither of those numbers is prime.
Hope that helped! =)