In an examination, a student scored 4 marks for every correct answer and losses 1 mark for every wrong answer. A student attempted all the 200 question and scores 200 marks. Find the number of questions, he answer correctly.

Correct Answer: 80

Explanation:

Let the number of correct answer be x and number of wrong answer be y.
Then, 4x - y = 200 ..(i)
and x + y = 200 ..(ii)
On adding Eqs. (i) and (ii), we get
4x - y = 200
x + y = 200
----------------
5x = 400
∴ x = 80