Let the number of correct answers be x
Number of questions answered wrong are (180 – x)
Total score when answered right = 4x
Marks deducted when answered wrong = 1(180 – x) = 180 – x
So,
4x – (180 – x) = 450
4x – 180 + x = 450
5x = 450 + 180
5x = 630
x = 630/5
= 126
∴ 126 questions he answered correctly.