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Pipes and Cistern - General Aptitude

Q1:

Two pipes A and B can fill a tank in 15 minutes and 20 minutes respectively. Both the pipes are opened together but after 4 minutes, pipe A is turned off. What is the total time required to fill the tank?

A 10 min. 20 sec.

B 11 min. 45 sec.

C 12 min. 30 sec.

D 14 min. 40 sec.

ANS:D - 14 min. 40 sec.

Part filled in 4 minutes = 4 1 + 1 = 7 .
15 20 15
Remaining part = 1 - 7 = 8 .
15 15
Part filled by B in 1 minute = 1
20
1 : 8 :: 1 : x
20 15
x = 8 x 1 x 20 = 10 2 min = 10 min. 40 sec.
15 3
The tank will be full in (4 min. + 10 min. + 40 sec.) = 14 min. 40 sec.