In an examination, a student scores 4 marks for every correct answer and loses 1 mark for every wrong answer. A student attempted all 200 questions and scored 500 marks. What is the number of questions he answered correctly?

Let the number of correct answers be c.
Since the student attempted all 200 questions, the number of wrong answers will be (200−c).

According to the marking scheme:

Total Marks=4×(Correct answers)−1×(Wrong answers)
500=4c−1(200−c)
Solving the linear equation:

500=4c−200+c
500+200=5c
700=5c
c=
5
700
​
=140
Thus, the number of questions answered correctly is 140.