a) 7.5 hr
b) 8 hr
c) 8.5 hr
d) 10 hr
Answer: a
Explanation: Work done by all the tanks working together in 1 hour,
$$ \Rightarrow \frac{1}{{10}} + \frac{1}{{12}} – \frac{1}{{20}} = \frac{2}{{15}}$$
Tank will be filled in $$\frac{{15}}{2}$$ = 7.5 hour.
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