A tank can be filled by pipe 'A' in 2 hours and pipe B in 6 hours. At 10 AM A was opened. At what time will the tank be filled if pipe B is opened at 11 AM?

11:45

Pipe A works alone for 1 hour . so, work done by A =  3 × 1  = 3

then remaining work = 6 - 3 = 3

at 11 AM , B also opened . Now A and B both worked together 

total efficiency of ( A + B ) =  3 + 1 = 4 

 As, Total work = efficiency × time  

 so , time take by ( A + B ) = \(\frac{3}{4}\)  

Tank will fill at → \(\frac{3}{4}\) + 11 AM

                    = 11:45 AM