According to the meaning of the question: 10*X-5*( 10-X)=50.

X=20/3

It can be seen that he didn't do the problem, that is, he didn't score or deduct points.

A: 1. He may have only done five questions, and they were all right.

He may have done eight questions, six right and two wrong.

Other assumptions are not valid.

So, he answered five or six questions correctly.