In an examination a student scores 1 mark for each correct answer and 0.25marks are deducted for each wrong answer. If he attempts all 120 questions and secures 90 marks the number of question he attempts correctly is?

Assuming 90 marks is a score (and not a percentage) 
R-right answers
W-wrong 
Total questions: R+W=120 
Score: R-0.25W=90 

System 
R+W=120
R-0.25W=90 

Subtracting equations leads to 
(R+W)-(R-0.25W)=120-90 
then 
1.25W=30 
W=24 
so that R=120-24=96 

96 correct 


:)