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