Assuming all the answers are correct, the score should be 20×5= 100 (points).

But the actual score: 100-82= 18 (points),

There are fewer questions than incorrect answers: 5+ 1=6 (points),

Incorrect answer: 18÷(5+ 1)=3 (Dao),

Correct answer: 20-3= 17 (Dao),

A: He answered 17 correctly.

So the answer is: 17.