Question 37308
Good problem.  Never heard that one before.  If you look at the list of primes,
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, etc.,
you find that differences of 1, 2, 3, 4, 5, 6, 8, 9, 10, etc., exist but no 7.
And because all later primes are odd numbers, a difference of seven will not exist there either.
Thus the answer is 7.  Good problem.