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**Quantitative Aptitude**Offline Downloads:
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Pipe and cistern concept
Pipes and cisterns are almost the same as time and work problems. There are
two kinds of pipes.
1 Inlet pipe- it fills the tank.
If a pipe fills a tank in x hours.
1
th
x
Then in 1 hours, it will fill
part of the tank.
2 outlet pipe- it impties the tank. If a pipe impties a tank in y hours, then in 1
1
th
y
hour, it will empty
part of the tank.
We use the same LCM method to solve the questions as we use in time an work
problems. But one thing should be kept in mind that the work of an outlet pipe
is negative.
Let’s understand a few problems.
Q.1 Pipe A can fill a tank in 10 minutes and Pipe B can fill it in 5 minutes.
Another Pipe can empty it in 15 minutes. If they are opened simultaneously then
how much time will be required to fill the tank?
SolutionA 10
B5
C 15
L.C.M= 30
In 1 min. A fills
30
30
3l , B fills 6l
10
5
and C empties
l
30
2l
15
Tank filled in 1 hr= 3+6-2=7
So total time to fill the tank= min
Q.2 Two pipes can fill a cistern in 1 hour and 75 minutes respectively. there is
also an outlet C. if all the three pipes are opened together. The tank is full in 50
minutes. How much time will be taken by C to empty the full tank?
Solution-
Pipes and cisterns
Page 1

A 60
B 75
L.C.M. 300
In 1 min, A fills
300
5l
60
B fills
300
4l
75
Hence in 50 min, A and B can fill
Capacity of tank- 300
tank filled in 1 min= 5+4=9
l
9 50 450l
l
l
150 of tank was emptied by pipe c in 50 minutes.
Hence in 1 minutes, Pipe C will empty
So to empty full tank of 300
150
3l
50
l
300
3
It will take
= 100 minutes
Q.3 Two, taps A and B can fill a tank in 10 hours and 8 hours respectively . If
both the taps are opened simultaneously them how many hours before tap B
should be closed so that the tank fills in 5 hours.?
SolutionA 10
B8
L.C.M 40
Hence in 1 hour, A will fill
A remains open fill end.
So A fills
5 4 20l
40
4l
10
and B will fill
40
5l
8
of the tank.
( in 5 hours)
l
Hence remaining 20 must be filled by B.
l
20
4hours
5
So to fill 20 , B takes
Q.4 Pipe A and B fill a cistern in 9 hours and 18 hours respectively but a third
pipe C can empty the tank in 27 hours. If all the pipes are opened
Pipes and cisterns
Page 2

simultaneously but after 4 hours pipe C is closed, then calculate the time in
which the tank will be filled?
SolutionA9
B 18
C 27
L.C.M 54
l
l
l
In 1 hour, A fill 6 , B fill 3 and C empties 2 .
C is open only for 4 hours.
Tank filled in 4 house =
(6 3 2) 4 28l
Tank to be filled after 4 hours =54-28=26
l
26
26
hours
63 9
l
26 will be filled by A and B in
5 Pipe A and B can fill a cistern in 12 minutes and 15 minutes respectively but a
third pipe C can empty it in 6 minutes. The first two pipes are kept open for 5
minutes and then pipe C is also opened. Find the time taken to empty the
cistern.
SolA 12
B 15
C 6
L.C.M 60
l
l
l
In 1 hour, A fills 5 , B fills 4 and empties 10 , A and b are open for initial 5
l
minutes, which fill (5+4) =45 of tank.
Now C is also open.
So in 1 minute = 5+4-10=-1 L of tank is emptied.
l
So empty 45 , it will take 45 minutes.
Pipes and cisterns
Page 3

2
th
5
Q.6 A cistern is
filled. A pipe A can fill it in 10 minutes and Pipe b can
empty it in 6 minutes. If both pipes are opened simultaneously. How long will
they take to empty the cistern.
SolA 10
B 6
L.C.M 30
l
l
In 1 minute, A fills 3 . and B empties 5 of tank. But tank is
2
5
filled
2
30 12l
5
When both A and B are opened= 3-5=2
l
l
2 of tank is emptied in 1 minutes.
12
2
l
12 of tank is emptied in
6 minutes
7. Pipe A and B can fill a cistern in 10 hours and 15 hours respectively. Both the
pipes are open together to fill the tank, but when the tank is
develop in the tank which empties
by Pipe A and B to fill the cistern.
Solution-
1
th
12
l
l
l
Both are open so in 1 hour (3+2)=5 of tank is filled.
Pipes and cisterns
filled, a tank
of water of the tank. Find the time taken
A 10
B 15
L.C.M. 30
In 1 hour, A fills 3 and B fills 2
th
6
Page 4

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