a) 20 minutes
b) 25 minutes
c) 30 minutes
d) 45 minutes

Answer: d
Explanation:

(A + B)’s filling (2 + 1) = 3 units/min
In 5 minutes they will fill 3 × 5 = 15 units
Remaining capacity = 60 – 15 = 45 units
Second pipe (B) fills it in
$$\eqalign{ & = \frac{{{\text{Remaining capacity}}}}{{{\text{efficiency of B}}}} \cr & = \frac{{45}}{1} \cr & = \,45\,{\text{minutes}} \cr} $$