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?

Question: 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?

Solution:
Part filled in 4 minutes = 4(1/15 + 1/20) = 7/15
Remaining part =(1 - 7/15) = 8/15

1/20 Part filled by B in 1 minute
∴ Full Part filled by B in 20 minutes
∴ 8/15 Part filled by B in (20 × 8)/15 minutes
= 10 minutes 40 seconds

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