Two pipes can fill a tank in 30 minutes and 45 minutes respectively. If both pipes are opened at the same time, how long should pipe B be closed so that the tank is filled in 20 minutes?

Correct answer marked below

5 minutes

Correct

7 minutes

10 minutes

11 minutes

12 minutes

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Explanation

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Official Explanation

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Pipe A operates for 20 minutes, so the amount of work done in that time is given by

\frac{20}{30} = \frac{2}{3}

. This means that the remaining work to fill the tank is

1 - \frac{2}{3} = \frac{1}{3}

. Pipe B can…

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What this question tests

Your ability to combine individual work rates, usually by working with the fraction of a job completed per unit of time.

Adding the times instead of the rates, or forgetting to take the reciprocal when combining rates.