15. Jenny calculates the average of five different prime numbers. Her answer is an integer. What is the smallest possible integer she could have obtained?
A 2
B 5
C 6
D 12
E 30
SOLUTION
C

For the average to be an integer the sum of 5 primes must be divisible by 5 . The sum of the first 5 primes is $2+3+5+7+11=28$ which is not a multiple of 5 . However, the next smallest sum of five primes is $2+3+5+7+13=30$ and the average of these five is 6 .