Let x be the number of correct answers and y be the number of wrong answers.
As per the question,
3x - 1y = 80 ....(1)
4x - 2y = 100 ....(2)
Multiplying equation (1) by 2,
6x - 2y = 160 ....(3)
Subtracting equation (2) from (3),
2x = 60
⇒ x = 30
∴ y = 3x - 80 = 10
So, total number of questions = 10+30 = 40