In an examination, a student attempted 15 questions correctly and secured 40 marks. If there were two types of questions (2 marks and 4 marks questions), how many questions of 2 marks did he attempt correctly?
(a)5
(b)10
(c)20
(d)40
Answer
Answer (as printed): B
Explanation
Suppose the student attempted $x$ questions of 2 marks correctly. Then, number of 4 marks questions attempted correctly $$\begin{aligned} & =(15-x) . \\ \therefore \quad 2 x+4(15-x) & =40 \Leftrightarrow 2 x+60-4 x=40 \\ & \Leftrightarrow 2 x=20 \Leftrightarrow x=10 . \end{aligned}$$
Explanation as extracted from the printed page; notation may be imperfect.