a) 12 minutes
b) 15 minutes
c) 25 minutes
d) 50 minutes
Answer: a
Explanation: Part filled by A in 1 minute = $$\frac{1}{{20}}$$
Part filled by B in 1 minute = $$\frac{1}{{30}}$$
Part filled by (A + B) in 1 minute
$$\eqalign{
& = {\frac{1}{{20}} + \frac{1}{{30}}} = \frac{1}{{12}} \cr} $$
Both pipes can fill the tank in 12 minutes
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