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Time & Work · Question 54 of 61

Question 54

MediumOriginal question

Pipes A and B can fill a tank in 30 and 45 minutes. Both are opened on an empty tank, and B is closed after some time. The tank is full exactly 20 minutes after the pipes were opened. For how many minutes was B open?

Options for question 54

Show the answer and explanation
CorrectNot quiteAnswer C

Correct answer: C

Explanation

  1. A runs all 20 minutes: 2030=23\frac{20}{30} = \frac{2}{3} of the tank.

  2. B filled the rest, 13\frac{1}{3}: time = 13×45=15\frac{1}{3} \times 45 = 15 minutes.

Worth noting

18 minutes is how long both together would take for the whole tank. 10 minutes uses A's 30 minutes instead of B's 45 when converting B's third.

Concept
Pipe closed partway: work done by each pipe
Typical time
70 s
Your time
—
The rule this testsPipes and cisterns
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Ten questions from Time & Work, at your own pace.

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

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