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Boats & Streams, Pipes & Cisterns · Question 60 of 85

The tank is full, and only A and C are open.

Question 60

MediumOriginal 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.

The tank is full, and only A and C are open. How long does the tank take to empty?

Options for question 60

Show the answer and explanation
CorrectNot quiteAnswer C

Correct answer: C

Explanation

  1. C empties faster than A fills: net emptying rate =19−112=4−336=136= \frac{1}{9} - \frac{1}{12} = \frac{4 - 3}{36} = \frac{1}{36}.

  2. So 36 hours.

Worth noting

3 hours subtracts the times, 12−912 - 9; it is the rates that subtract.

Concept
Emptying a tank against an inlet
Typical time
120 s
Your time
—
The rule this testsPipes filling a tank
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