Don't answer a question, * * * answer fifteen questions. If all the pairs are correct, you can get 6X 15=90 points. A wrong question 6+2= 8 points less. So he can only make two mistakes at most. He should at least answer 15-2= 13.

Check calculation:

For the question 13, the score is13x6 = 78; If two questions are wrong, deduct 2X2=4, and finally score 78-4 = 74>；; 70 points.