Three pipes P, Q and R can separately fill a cistern in 4, 8 and 12 hours respectively. Another pipe S can empty the completely filled cistern in 10 hours. Which of the following arrangements will fill the empty cistern in less time than others?

Answer: Option D

$$\begin{array}{rl}& \text{(Total Capacity)}\\ & \text{120}\\ & \overline{\downarrow \downarrow \downarrow \downarrow }\\ & \text{efficiency 30}1510-12\\ & \text{hours}\to \underset{\left(\text{P}\right)}{\overset{\downarrow }{4}}\underset{\left(\text{Q}\right)}{\overset{\downarrow }{8}}\underset{\left(\text{R}\right)}{\overset{\downarrow }{12}}\underset{\left(\text{S}\right)}{\overset{\downarrow }{10}}\end{array}$$
In order to fill the cistern in less time.
So, efficiency of filling should be more
now, check all options
(A) → Q efficiency 15 units/hr
(B) → (P + R - S) efficiency
          = 30 + 10 - 12 = 28 units/hr
(C) → (P - S) efficiency
          = 30 - 12 = 18 units/hr
(D) → (P + Q - S) efficiency
          = 30 + 15 - 12 = 33 units/hr
Option (D) is answer
Since efficiency of option (D) is highest