A pipe can fill a tank in 6 hours and another pipe can empty the tank in 12 hours. If both the pipes are opened at the same time, the tank can be filled in

10 Hrs 12 Hrs 14 Hrs 16 Hrs

12 Hrs

The following chart has to be made first: Efficiency of A = 2 Efficiency of B = -1 Efficiency of A + B = 1 Thus, it will take 12 hours for the cistern to be filled: \(\frac{12 }{1 }\) = 12 hours