Two pipes can fill a tank in $20$ hours and $30$ hours, while a drain can empty it in $60$ hours. If all three are opened together, the tank fills in:

Question

Two pipes can fill a tank in $20$ hours and $30$ hours, while a drain can empty it in $60$ hours. If all three are opened together, the tank fills in:

Option A

12 hours

Option B — Correct answer

15 hours

Correct choice.

Option C

18 hours

Option D

20 hours

Explanation

Net rate $=\frac1{20}+\frac1{30}-\frac1{60}=\frac{3+2-1}{60}=\frac4{60}=\frac1{15}$. So the tank fills in $15$ hours.

Chapter Mock 7: Time and Work, Pipe and Cistern