If p and p + 2 are both prime and p is greater than 3, then p + 1 is always a multiple of:

p is odd, so p + 1 is even. Also, of the three numbers p, p + 1, p + 2 exactly one is a multiple of 3, and it cannot be p or p + 2 because both are primes bigger than 3 — so it must be p + 1. Being a multiple of both 2 and 3, p + 1 is a multiple of 6. The examples confirm it: 5 and 7 give 6, 11 and 13 give 12, 17 and 19 give 18, 29 and 31 give 30. Answering 4 fails at 6 itself, and 5 or 10 fail at 12.