An overhead tank can be filled by pipe P in 12 hours and by pipe Q in 15 hours. If both pipes are opened together on an empty tank, how long will it take to fill?

6 hours 40 minutesCorrect answerYour answer7 hours 30 minutesCorrect answerYour answer13 hours 30 minutesCorrect answerYour answer27 hoursCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer A

Correct answer: A

Together: $\frac{1}{12} + \frac{1}{15} = \frac{5+4}{60} = \frac{9}{60} = \frac{3}{20}$ per hour.
Time = $\frac{20}{3}$ hours = 6 hours 40 minutes.

Worth noting

27 hours adds the two times and 13 hours 30 minutes averages them. Two pipes must fill faster than the quicker pipe alone.

Concept: Two inlet pipes together
Typical timeTypical: 40 s
Your timeYours: —

The rule this testsPipes and cisterns

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Ten questions from Time & Work, at your own pace.

An overhead tank can be filled by pipe P in 12 hours and by pipe Q in 15 hours. If both pipes are opened together on an empty tank, how long will it take to fill?

Explanation