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Boats & Streams, Pipes & Cisterns · Question 49 of 85

How long do A and B together take to fill the empty tank, with C shut?

Question 49

EasyOriginal question

A tank has two inlet pipes, A, which fills it in 12 hours, and B, which fills it in 18 hours, and an outlet C, which empties the full tank in 9 hours.

How long do A and B together take to fill the empty tank, with C shut?

Options for question 49

Show the answer and explanation
CorrectNot quiteAnswer A

Correct answer: A

Explanation

  1. 112+118=3+236=536\frac{1}{12} + \frac{1}{18} = \frac{3 + 2}{36} = \frac{5}{36} of the tank an hour.

  2. 365=7.2\frac{36}{5} = 7.2 hours.

Worth noting

15 hours averages the two times.

Concept
Two pipes together
Typical time
90 s
Your time
—
The rule this testsPipes filling a tank
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