Assuming he is all right, he should get a perfect score of 120.

If 1 is wrong, not only will you not get 8 points, but you will also get 4 points backwards.

The actual score will lose 8+4= 12 points.

And his actual score is 84 points, which is less than the full mark 120-84=36 points.

36÷ 12=3 questions.

In other words, he only made three mistakes and got the right answer 15-3= 12.

Equation method: If you answer the X question correctly, you will answer the wrong (15-x) question.

8x-4( 15-x)=84

The solution is x= 12.