All three pipes are opened on the empty tank.

Question 59

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.

All three pipes are opened on the empty tank. How long does it take to fill?

7.2 hoursCorrect answerYour answer9 hoursCorrect answerYour answer18 hoursCorrect answerYour answer36 hoursCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer D

Correct answer: D

Net rate $= \frac{1}{12} + \frac{1}{18} - \frac{1}{9} = \frac{3 + 2 - 4}{36} = \frac{1}{36}$ .
So 36 hours.

Worth noting

7.2 hours leaves the outlet out.

Concept: Two taps and an outlet
Typical timeTypical: 120 s
Your timeYours: —

The rule this testsPipes filling a tank

Practise this chapter →

Ten questions from Boats & Streams, Pipes & Cisterns, at your own pace.

Practise this chapter →

Ten questions from Boats & Streams, Pipes & Cisterns, at your own pace.

All three pipes are opened on the empty tank.

Question 59

Explanation