punindya wrote:
If (n+7)/2 is a multiple of 4, then n+7 is a multiple of 8.
All multiples of 8 are even, and the sum of two numbers is even only if both are odd or both are even. Since 7 is odd, n is odd too.
n can be prime. To check this easily, start with multiples of 8. If n+7=8, n=1. If n+7=16, n=9. If n+7=24, n=17, which is a prime number.
Also, as seen above, if n+7=16, n=9, which is a multiple of 3.
You cannot use n=17 as your prime, since if n=17, 17+7 = 24, which is divisible by 6. The prompt states that n+7 is not divisible by 6. Luckily, later in the list, we see n=73 IS a prime, so the answer is still that primes are possible.