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?

Answer: Option D

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)
$$\begin{array}{c}\text{Pumps 4}:3\text{hfill}\\ \text{Days}\text{1}:2\text{hfill}\end{array}\}::8:x$$
$$\begin{array}{rl}& \therefore 4\times 1\times x=3\times 2\times 8\\ & \Rightarrow x=\frac{3\times 2\times 8}{4}\\ & \Rightarrow x=12\end{array}$$