# Aptitude Questions on Pipes-Cisterns

1. Two pipes A and B can fill a cistern in 20 and 30 minutes respectively, and a third pipe C can empty it in 40 minutes. How long will it take to fill the cistern if all the 3 pipes are opened at the same time?
A. 7 1/7 mins.
B. 15 1/7 mins.
C. 17 1/7 mins.
D. 19 1/7 mins.

B. 17 1/7 mins
1/20 + 1/30 - 1/40 = 7/120
=>120/7 = 17 1/7
3. Two taps can separately fill a cistern 10 minutes and 15 minutes respectively and when the waste pipe is open, they can together fill it in 18 minutes. The waste pipe can empty the full cistern in?
A. 7 mins.
B. 9 mins.
C. 13 mins.
D. 23 mins.

B. 9 mins
1/10 + 1/15 - 1/x = 1/18
x = 9
5. A cistern is normally filled in 8 hours but takes two hours longer to fill because of a leak in its bottom. If the cistern is full, the leak will empty it in?
A. 16 hrs
B. 20 hrs
C. 25 hrs
D. 40 hrs

D. 40 hrs
1/8 - 1/x = 1/10
x = 40
7. Two pipes can fill a tank in 18 minutes and 15 minutes. An outlet pipe can empty the tank in 45 minutes. If all the pipes are opened when the tank is empty, then how many minutes will it take to fill the tank?
A. 9 mins.
B. 10 mins.
C. 11 mins.
D. 12 mins.

B. 10 mins
Part of the filled by all the three pipes in one minute
= 1/18 + 1/15 - 1/45 = (5 + 6 - 2)/90 = 9/90 = 1/10
So, the tank becomes full in 10 minutes
9. Pipe A can fill a tank in 16 minutes and pipe B cam empty it in 24 minutes. If both the pipes are opened together after how many minutes should pipe B be closed, so that the tank is filled in 30 minutes?
A. 19 mins.
B. 20 mins.
C. 21 mins.
D. 22 mins.

C. 21 mins
Let the pipe B be closed after x minutes.
30/16 - x/24 = 1 => x/24 = 30/16 - 1 = 14/16
=> x = 14/16 * 24 = 21
11. Three pipes A, B and C can fill a tank from empty to full in 30 minutes, 20 minutes and 10 minutes respectively. When the tank is empty, all the three pipes are opened. A, B and C discharge chemical solutions P, Q and R respectively. What is the proportion of solution R in the liquid in the tank after 3 minutes?
A. 5/11
B. 6/11
C. 7/11
D. 8/11

B. 6/11
Part filled by (A + B + C) in 3 minutes = 3(1/30 + 1/20 + 1/10) = 11/20
Part filled by C in 3 minutes = 3/10
Required ratio = 3/10 * 20/11 = 6/11
13. Three pipes A, B and C can fill a tank from empty to full in 30 minutes, 20 minutes, and 10 minutes respectively. When the tank is empty, all the three pipes are opened. A, B and C discharge chemical solutions P,Q and R respectively. What is the proportion of the solution R in the liquid in the tank after 3 minutes?
A. 5/11
B. 6/11
C. 7/11
D. 8/11

B. 6/11
Part filled by (A + B + C) in 3 minutes = 3 (1 / 30 + 1 / 20 + 1 / 10) = (3 * 11 / 60) = 11 / 20
Part filled by C in 3 minutes = 3 / 10
Therefore, required ratio = (3 / 10 * 20 / 11) = 6/11
15. Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in:
A. 1 13/17 hrs
B. 2 8/11 hrs
C. 3 9/17 hrs
D. 4 1/2 hrs

C. 3 9/17 hrs
Net part filled in 1 hour (1 / 5 + 1 / 6 - 1 / 12) = 17 / 60
Therefore, the tank will be full in 60 / 17 hours i.e., 3 9/17 hrs
17. A pump can fill a tank with water in 2 hours. Because of a leak, it took 2 1/3 hours to fill the tank. The leak can drain all the water of the tank in:
A. 10 hrs
B. 12 hrs
C. 14 hrs
D. 16 hrs

C. 14 hrs
Work done by the leak in 1 hour = (1/2 - 3/7) = 1/14
Leak will empty the tank in 14 hrs
19. Two pipes A and B can fill a cistern in 37 1/2 minutes and 45 minutes respectively. Both pipes are opened. The cistern will be filled in just half an hour, if the B is turned off after:
A. 6 mins.
B. 9 mins.
C. 12 mins.
D. 15 mins.