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?

Amount of water filled by pipe A in one minute = 1/15 Amount of water filled by pipe B in one minute = 1/20 So, both pipes were opened for 4 minutes. So, the amount of water filled by both pipes in 4 minutes = 4* (1/15+ 1/20) = 7/15 So, remaining amount = 8/15 Now, pipe A is turned off, So, this will be filled by pipe B. let's say in x minutes So, x*(1/20) = 8/15 X = 32/3 minutes it means 10 minutes 40 seconds So, total time taken = 4 minutes + 10 minutes 40 seconds = 14 minutes 40 seconds