Three taps A, B, C can fill an overhead tank in 4, 6 and 12 hours respectively. How long would the three taps take to fill the tank if all of them are opened together ?

Answer: Option A

(A + B + C)'s efficiency
= 3 + 2 + 1
= 6 units/hr
(A + B + C) can fill the tank in
$$\begin{array}{rl}& \text{ = }\frac{\text{Total Capacity}}{\text{Efficiency of}\left(\text{A + B + C}\right)}\\ & =\frac{12}{6}\\ & =2\text{ hours}\end{array}$$