Question 7

It appears that Agent P might be working for the enemy. But in order to get into the safe where he keeps his private documents, you have to enter a string of 8 numbers. You’ve got 7 of them but what’s the last number that will complete this sequence 13, 17, 31, 37, 71, 73, 79…?

get the answer? get the answer?
Clue?
This is a prime time to reverse your strategy.