Two pipes A and B can fill a tank in 15 min and 20 min respectively Both the pipes are opened togeth...........

Quant (SSC) Pipes&cistern

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

15 min 20 sec.

16 min 40 sec.

13 min 10 sec.

14 min 40 sec.

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Explanation:

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