a) 15 minutes
b) 20 minutes
c) 27.5 minutes
d) 30 minutes
Answer: d
Explanation:
$$\eqalign{
& {\text{Part}}\,{\text{filled}}\,{\text{by}}\,(A + B)\,{\text{in}}\,{\text{1}}\,{\text{minute}} \cr
& = {\frac{1}{{60}} + \frac{1}{{40}}} = \frac{1}{{24}} \cr
& {\text{Suppose}}\,{\text{the}}\,{\text{tank}}\,{\text{is}}\,{\text{filled}}\,{\text{in}}\,x\,{\text{minutes}} \cr
& {\text{Then}},\,\frac{x}{2}\left( {\frac{1}{{24}} + \frac{1}{{40}}} \right) = 1 \cr
& \frac{x}{2} \times \frac{1}{{15}} = 1 \cr
& x = 30\,\text{minutes} \cr} $$
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