If 9 examiners can examine a certain number of answer books in 12 days, working 5 hours a day; for how many hours a day would 4 examiners have to work in order to examine twice the number of answer books in 30 days?
(a)6
(b)8
(c)9
(d)10
Answer
Answer (as printed): C
Explanation
Let the required number of working hours per day be $x$. Less examiners, More working hours per day (Indirect Proportion). More days, Less working hours per day (Indirect Proportion). More answer books, More working hours per day (Direct Proportion). Examiners $4:9$, Days $30:12$, Answer books $1:2$ :: $5:x$. $\therefore (4 \times 30 \times 1 \times x) = (9 \times 12 \times 2 \times 5) \Leftrightarrow 120x = 1080 \Leftrightarrow x = 9$.
Explanation as extracted from the printed page; notation may be imperfect.