Not sure if this is how your teacher teaches but here’s my answer

Above method may not work for pri sch, as thr students have not learnt simultaneous equations. P5 students likely have not learnt algebra either.

There are 2 non-algebraic ways of solving this question. Either by assumption or guess and check, which I've done both

If you don't understand the assumption method, you can do the guess and check. Since it's not too long either, so rougly same amount of time to use guess and check.

For assumption method, first we assume he gets all the questions correct, meaning he would get 100 marks.

However,if he gets one question wrong, not only does he not get the 5 marks, he loses 3 marks as well. So in total, for every wrong question, he loses 8 marks.

Since he got only 84 marks, we calculate the total number of marks he lost. 16 marks = 2 wrong questions. Since he loses 8 marks for each wrong question.

This is assumption method (B) . It is useful to solve this type of question besides guessing method. There are steps by steps video presentation from https://youtu.be/t27lYHwIiag to help you.