If you get 60 points, you lose 40 points, that is, you made five mistakes and got 15.
Only 25- 15 = 10 people answered the first question correctly.
Only 23- 15 = 8 people answered the second question correctly.
15 people did it right.
10+8+ 15 = 33 people answered at least one question correctly.
36-33 = 3 people are wrong.