In an examination, a student gets 4 marks for each correct answer and loses 1 mark for each wrong answer. A student attempted all 200 questions and scored 400 marks. The number of questions he answered correctly is:

Solving the marking scheme gives 120 correct answers.

Let the correct answers be c and the wrong answers 200 minus c, since all 200 were attempted.

Scoring gives 4c minus (200 minus c) equals 400.

This simplifies to 5c minus 200 equals 400, so 5c equals 600.

Dividing gives c equals 120 correct answers.

The remaining 80 wrong answers lose 80 marks, and 480 minus 80 equals 400, confirming the result.