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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?

10 min. 20 sec.
11 min. 45 sec.
12 min. 30 sec.
14 min. 40 sec.

Correct Answer: 14 min. 40 sec.

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 : 8/15 :: 1:x

x = (8/15*1*20) = $10 \frac{2}{3} m i n = 10 m i n 40 s e c$

The tank will be full in (4 min. + 10 min. + 40 sec.) = 14 min. 40 sec

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