Hang on, I lost this post. I retype.

I managed to get answer a. However I don’t know how to get question b answer. The answer sheet showed 25 questions

(b)

Observe that a change from WRONG to CORRECT means an increase in 7 marks, because you would have not lost the 2 marks which you would have in a WRONG answer, and even gain 5 marks because you get the question CORRECT.

Imagine there are four questions. I answered all four correctly. I get 4 x 5 = 20 points. You answered three correctly and one wrongly. You get 3 x 5 - 1 x 2 = 15 - 2 = 13 points.

That's a difference of 7 points, as I explained above.

Now, because Student D obtains 21 points more than Student C, and each "block" of 7 points means a change in one WRONG answer to one CORRECT answer, there must have been 21/7 = 3 of such "blocks".

In other words, Student D had 3 more CORRECT answers but 3 fewer WRONG answers than Student C.

Student D must have answered 22 + 3 = 25 questions correctly (and 8 - 3 = 5 anwers wrongly).

Now we check these values.

Student C's final score
= 22 x 5 - 8 x 2
= 110 - 16
= 94 points

Student D's final score
= 25 x 5 - 5 x 2
= 125 - 10
= 115 points

and their difference is 115 - 94 = 21 points.

Ohhh!! Thank you for answering this so promptly! :)