Questions that were answered
= ⅝ x 240
= 150

Assume first that all questions were answered correctly.

Marks awarded = 150 x 4 marks
= 600 marks

Now, if one correct answer was changed to a wrong answer, we must deduct the 4 marks that were awarded. And we must deduct another 3 marks since it's now a wrong answer.

Overall , we deduct 7 marks for each correct answer that was changed to a wrong answer.

600 marks - 439 marks = 161 marks

Since 161 marks have to deducted to get to 439,

161 marks ÷ 7 marks = 23

23 correct answers have to be changed to wrong answers to get to 439 marks.

This actually means there were 23 wrong answers in the first place.

Number of correct answers
= 150 - 23
= 127

So can only do by guess & check, no other shorter ways? Thank you.

The above is the assumption method. It is faster than guess and check.

Thank you very much. I got it.

Hi Cindy, besides guess and check method, another method is called “ Assumption method B”, you can learn how to solve from https://youtu.be/t27lYHwIiag.

You're welcome, Cindy. Glad you got it!

Thank you Henry.

Your are really amazing guys out there, thank you all!