Pipe A can fill an empty tank in 6 hours while along with Pipe B it can fill it up in 4 hours. Only Pipe B is on for an hour after which Pipe A is also tured on. How much time will it take in all to fill up the tank?

Pipe A can fill the whole tank in 6 hours
Pipe B can fill the whole tank in
(1/b)=(1/4)-(1/6)
b=12 hours
LCM of 6 and 12 is 12 units
Each hour A can fill 2 units and B can fill 1 unit of water in tank
Only B was opened for 1 hour so 1 unit will be filled and so 11 units will be left
Both A and B can fill 3 units per hour and so they can fill the 11 units in (11/3)hours
Total time 1+(11/3)=14/3 hours i.e 4 hrs 40 minutes