pipes and cisterns - vidyarthiplus
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PIPES AND CISTERNS
1. Two taps A and B can fill a tank in 10 hours and 15 hours and 15
hours respectively. If both the taps are opened together, the tank will
be full in ?
Ans:6hrs
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
If A can do a piece of work in x hours and B can do a piece of work in
y hours. Then A and B together will do the work in yxxy+ hours
Answer with Explanation:
Time taken by A = 10 hours
Time taken by B = 15 hours
Time taken by A and B =10151015
+X
=25
1015X= 6 hours.
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2. To fill a cistern, pipes A, B and C take 20 minutes, 15 minute and 12
minutes respectively. The time in minutes that the three pipes together
will take to fill the cistern is:
Ans:5
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
A can do a piece of work in x minutes
B can do a piece of work in y minutes
C can do a piece of work in z minutes
Minute’s work of each of the three is 1/x +1/y + 1/z
Answer with Explanation:
1 minutes work of each of the three pipes
= 1/20 + 1/15 + 1/12
=60
543 ++=
6012
= 1/5
= 5 minutes
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3. Two pipes can fill a tank in 10 hours and 12 hours resp. while third
pipe empties the full tank in 20 hours. If all the three pipes operate
simultaneously, in how much time the tank will be filled?
Ans;7 hrs 30 mins
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Pipe A can fill a tank in x hours and pipe B can fill a tank in y hours and
pipe C can empty it in z hours. If all the pipes are operate simultaneously,
1 hours of work of each of the three pipes = 1/x +1/y - 1/z
Answer with Explanation:
1 minutes work of each of the three pipes
= 1/10 + 1/12 - 1/20
=60
356 −+=
608
=152
= 15/2 hours = 7 ½ hour
= 7 hours 30 minutes.
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4. A cistern can be filled in 9 hours but it takes 10 hours due to a leak in
its bottom. If the cistern is full, then the time that the leak will take to
empty it is:
Ans:90 hrs
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
If A can fill a tank in x hours and B can fill a tank in y hours. Then the
tank will be filled (empty) in yxxy− hours
Answer with Explanation:
Time taken by A = 9 hours
Time taken by B = 10 hours
Time taken by A and B =910
109−x
= 90 hours.
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5. an electric pump can fill a tank in 3 hours. Because of a leak in the
tank, it took 3 ½ hours to fill the tank. The leak can drain out of all the
water of the tank in:
Ans:21 hrs
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
If A can fill a tank in x hours and B can fill a tank in y hours. Then the tank
will be filled (empty) in yxxy− hours
Answer with Explanation:
Time taken = 3 hours
Time taken = 3 ½ hours
Time taken by A and B =3
213
2133
−
x
=3
27
273
−
x
=2
67273
−
x
= 21 hours
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6. Taps A and B can fill a bucket in 12 minutes and 15 minutes
respectively. If both are opened and A is closed after 3 minutes. How
much further time would it take for B to fill the bucket?
Ans 8 min 15 sec
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Total work (to fill or empty) = I part + II part =1
Find I st part and subtract it from ‘1’ , then multiply the required
section value to find the answer.
Answer with Explanation:
For 3 minutes (I part)
= 3
+
151
121
= 3
+
12151215X
= 3
203
= 9/20
Remaining part
= 1- 9/20 = 11/20
Tap B fill the bucket in
=2011
-15
= 33/4 = 8 ¼ hours
= 8 hours 15 seconds.
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7. A tank can be filled by a tap in 20 minutes and by another tap in 60
mintues. Both taps are kept open for 10 mintues and then the first tap
is shut off. After this, the tank will be completely filled in:
Ans:20 min
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Total work (to fill or empty) = I part + II part =1
Answer with Explanation:
For 10 minutes (I part)
= 10
+
601
201
= 10
604
= 4/6 = 2/3
Remaining part
= 1- ( I part)
= 1-2/3
= 1/3
The tank will be complete by filled in (B only, after 10 minutes)
= 1/3 X 60
= 20 minutes.
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8. If two pipes function simultaneously the reservoir will be filled in 12
hours, one pipe fills the reservoir 10 hours faster than the other. How
many hours it takes the second pipe to fill the reservoir?
Ans:30 hours
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Taken unknown value as x and use yxxy+
Answer with Explanation:
Let the reservoir be filled by forst pipe in x hours, therefore the second pipe
will fill it in (x+10) hours.∴ 1/x + 1/ x+10 = 1/12
+
++)10(
10xx
xx= 1/12
12 (10+2x) = x2 +10x
x2 +10x – 24x – 120 = 0
x2 – 14x – 120 = 0
(x-20) (x+6) = 0∴ x = 20 (x should nor –ve)∴ The second pipe can take (x+10) = 20 + 10 = 30 hours, to fill the
reservoir.
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9. One tap can fill a cistern in 2 hours and another tap can empty the
cistern in 3 hours. How long will they take to fill the cistern if both the
taps are opened?
Ans: 6hours
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Pipe A can fill a tank in ‘x’ hours and pipe B can empty it in ‘y’ hours. If
both the pipes opened together, the tank will be filled in yxxy− hours.
Answer with Explanation:
Time taken to fill the tank = 2 hours
Time taken to empty the tank = 3 hours
Time taken to fill the cistern =1332
−X
= 6 hours.
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10. 12 buckets of water fill a taken when the capacity of each bucked is
13.5 liter. How many buckets will be needed to fill the same tank, if
the capacity of each bucket is 9 litres ?
Ans:18
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Here the tank is same
Number of buckets X capacity of bucket = Number of buckets X
capacity of bucket
Answer with Explanation:
The capacity of the tank is same,
N1 X W1 = N2X W2
N1 = No. of Buckets W1 = Capacity of liters in the buckets
N2 = No. of Buckets W2 = Capacity of liters in the buckets
= 12 X 13.5 = N2 X9
= N2 =9
5.1312X= 54/3
= N2 = 18
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11. Bucket p has thrice the capacity as bucket Q. It takes 60 turns for
bucket P to fill the empty drum. How many turns it will take for both
the buckets p and q having each turn together to fill the empty drum:
Answer:45
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
= yxxy+
Answer with Explanation:
Bucket P has thrice the capacity as bucket Q
Q= 3 P
P takes 60 turns
Q takes = 180 turns
P and Q takes =1806060180
+X
=240
60180X
= 45 turns.
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12. Two pipes A and B fill a tank in 15 hours and 20 hours respectively
while a third pipe c can empty the full tank in 25 hours. All the three
pipes are opened in the beginning, after 10 hours, c is closed. In how
much time, will the tank be full ? ans:12 hrs
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
1/x +1/y - 1/z Part I and Part II = 1
Answer with Explanation:
A B C
15 20 -25
For 10 hours = 10
−+
251
201
151
= 10
−+
30012
30015
30020
= 10
30023
= 23/30 ( Part I)
Remaining part =1- 23/30 = 7/30
(A+B)’s 1 hour work
= 1/15 + 1/20 = 4/60 + 3/60
= 7/60
Therefore = 7/30 X 60/7 = 2 hours
Hence 10+2 = 12 hours
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13. Two pipes A and B and can fill a cistern in 12 minutes and 15
minutes respectively but a third pipe c can empty the full tank in 6
mintues. A and B are kept open for 5 minutes in the beginning and
then c is also opened. In what time is the cistern emptied …..
Ans:45 mints
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
1/x +1/y - 1/z
Answer with Explanation:
A B C
12 15 -6
For 5 minutes = 5
+
151
121
= 5
18027
= ¾
When all the three pipes are opened = 1/12 + 1/15 – 1/6
= 15/180 + 12/180 – 30/180
= -3/180 = -1/60 (-means emptying)
Empty the cistern in 1 min is 1/60
Therefore ¾ will emptied in ¾ X 60 = 45 minutes.
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14. Three pipes A, B and C can fill a tank in 6 hours. After working at it
together for 2hours, C is closed and A and B can fill the remaining
part in 7 hours. The number of hours taken by C alone to fill the
cistern is:
Answer:14
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
C’s work = (A+B+C)’s work – (A+B)’s work
Answer with Explanation:
A+B+C = 1/6
For 2 hours = 2 (1/6) = 1/3
Remaining = 2/3
(A+B)’s 7 hours work = 2/3
(A+B)’s 1 hours work = 2/21
C’s 1 hour work = (A+B+C)’s work – (A+B)’s work
= 1/6 -2/21
= 9/126 = 1/14
C alone can fill the tank in 14 hours
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15. A leak in the bottom of a tank can empty the full tank in 8 hours. An
inlet pipe fills water are the rate of 6 liters a minute. When the tank is
full, the inlet is opened and due to the leak, the tank is empty in 12
hours. How many litres does the cistern hold ?
Ans: 8,640
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Work done = 1/x – 1/y
Time X number of liters
Answer with Explanation:
Time taken to fill the tank is 1/8
Time taken to empty the taken is 1/12
Work done by the inlet in 1 hour =
+
121
81
=812812X−
=812
4X = 1/24
Work done by the inlet in 1 minute = 1/24 X 1/60 = 1/1440
Therefore Volume = 1440 X6 liters
= 8640 liters
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16. A leak in the bottom of a tank can empty the full tank in 6 hours. An
inlet pipe fills water at the rate of 4 litres a minute. When the tank is
full the inlet is opened and due to the leak the tank is empty in 8
hours. The capacity of the tank in litres is : Ans 5760.
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Work done = 1/x – 1/y
Time X number of liters
Answer with Explanation:
Time taken to fill the tank is 1/6 hour (Time taken to empty = time taken to
full)
Time taken to empty the tank is 1/8
Work done by the inlet in 1 hour = (1/6 – 1/8)
=8668X−
= 2 / 48 = 1/24
For 1 minute = 1/24X1/60 = 1/1440
An inlet pipe fill water at the rate of 4 liters a minute = 1440 X 4 = 5760
liters
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17. A tap can fill a tank in 16 minutes and another can empty it in 8
minutes. If the tank is already half-full and both tanks are opened
together, the tank will be :
Ans: emptied in 8 min
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Work done = 1/x – 1/y
Answer with Explanation:
Time taken to fill the tank in = 1/16 min
Time taken to empty the tank in = 1/8 min∴ Work done by both in 1 min = (1/16 – 1/8)
=816
168X
−= -8 / 16X8
= -1/16 (-ve means empty)
Now full tank will be emptied by them in 16 min
Half tank will be emptied in 8 min.
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18. A cistern has two taps which fill it in 12 min and 15 min respectively.
There is also a waste pipe in the cistern. When all the three are
opened, the empty cistern is full in 20 mintues. How long will waste
pipe take to empty the full cistern ?
Ans;10 min
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
C’s work = (A+B+C)’s work – (A+B)’s work
A + B = 1/x+1/y
Answer with Explanation:
To fill the cistern = (1/12 + 1/15) =1512
27X
= 3/20
Work done by waste pipe in 1 min
= work done by all the three taps – work done by two taps (fill)
= 1/20 – 3/20 = -2/20 = -1/10 (-ve means empty)
Waste pipe will empty the full cistern in 10 minutes.
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19. Two pipes A and B can fill a cistern in 12 minutes and 15 mint
respectively. If both the pipes are opened together, then after how
much time B should be closed so that the tank is full in 9 minutes?
Ans 4 min
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Let B be closed after x minutes.
I part + II part = 1
I part = Cistern filled by (A+B) in x min
II part = Cistern filled by A in (9-x) minutes
Answer with Explanation:
∴
+
161
121x + (9-x) X 1/12= 1
12129
16121216 xX
x −+
+
= 1
x1612
28x - x/12 = 1-9/12
124127 xxx − = 3/12
7x – 4x = 12
3x = 12
X = 4
So, B must be closed after 4 minutes.
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20. A water tank is two – fifth full. Pipe A can fill a tank in 10 minutes and
pipe B can empty it in 6 minutes. If both the pipes are open, how long will it
take to empty or fill the tank completely?
Answer : 6 minutes
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
AB/B-A
Answer with Explanation:
Water level in the tank is 2/5
Both the pipes are open = 610610
X−
=604
=151
Therefore 2/5 X15 = 6 minutes
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21. Pipes A and B take 20 and 30 minutes to fill a tank respectively. The 2
together will take :
Answer : 12
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
AB/A+ B
Answer with Explanation:
A = 20
B = 30
Together = A+B/AB
=30203020
X+
=3020
50X
=121
Therefore 12 minutes.
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22. 2 pipes A and B can fill a tank in 36 hours and 45 hours respectively. If
both are opened simultaneously, how much time will be taken to fill the
tank?
Answer : 20 hours
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
Time taken to fill the tank = A+B/AB
Answer with Explanation:
A = 36
B = 45
BAAB
+=
45364536
+X
=81
4536X
= 20 hours
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23. Two pipes A and B can fill a tank in 6 hours and 4 hours respectively.
If they are opened on alternate hours and if pipe A is opened first, in
how many hours, the tank shall be full ?
Ans: 5 hours
Difficulty Level : Easy Moderately easy Difficult
Basic Formula:
It’s an alternate hour’s problem
A’s 1hour work = 1/6
B’s 1 hour work = ¼
A + B means 2 hours of work
Answer with Explanation:
∴ (A+B)’s 2 hours work when opened alternately =
+
41
61
= 24
64 +
∴ (A+B)’s 4 hours work when opened alternately = 2 (5/12) = 5/6
Remaining = 1-5/6 =1/6∴ It is A’s turn and 1/6 part is filled by A in 1 hour∴ Total time taken to fill the tank = (4+1) hours
= 5 hours
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