Pipes A and B are filling pipes while pipe C is an emptying pipe. A and B can fill a tank in 72 and 90 minutes respectively. When all the three pipes are opened together, the tank gets filled in 2 Hours, A and B are opened together for 12 minutes, then closed and C is opened. After how much hours the tank will be empty?

18

Efficiency of A + B = ( 4 +5)  = 9

Tank fill in first 12 minutes = 9 × 12 = 108

Time by C to empty the tank = \(\frac{108}{9 - 3}\) = 18 minutes