Pipes A and B can fill a tank in 16 hours and 24 hours, respectively, whereas pipe C can empty the full tank in 40 hours. All three pipes are opened together, but pipe C is closed after 10 hours. After how many hours will the remaining part of the tank be filled?

2

Capacity of the tank = LCM of 16, 24, 40

Total efficiency of the system = 15 + 10 - 6 = 19

Capacity of the tank filled in 10 hrs = time x efficiency 

                                                   = 10 x 19 = 190

Capacity of the tank after this = 240 - 190 = 50

This will be filled by A and B in $\frac{50}{eff.\;of\;A\;+\;B}$

                                              = $\frac{50}{25}$ = 2hrs