3 pumps, working 8 hours a day, can empty a tank in 2 days. How many hours a day must 4 pumps work to empty the tank in 1 day?
(a)9
(b)10
(c)11
(d)12
Answer
Answer (as printed): D
Explanation
Let the required number of working hours per day be $x$. More pumps, Less working hours per day (Indirect Proportion) Less days, More working hours per day (Indirect Proportion) $\left.\begin{array}{ll}\text { Pumps } & 4: 3 \\ \text { Days } & 1: 2\end{array}\right\}:: 8: x$ $$\begin{aligned} & \therefore 4 \times 1 \times x=3 \times 2 \times 8 \\ & \Leftrightarrow x=\frac{3 \times 2 \times 8}{4} \Leftrightarrow x=12 . \end{aligned}$$
Explanation as extracted from the printed page; notation may be imperfect.