Question 44

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, The tank will be empty after:

Solution

Let the total work be 360 units.
($$\because$$ LCM of 72, 90 and (2hr = 120) is 360.)  
Efficiency of A = 360/72 = 5
Efficiency of B = 360/90 = 4
Efficiency of A, B and C = 360/120 = 3
Efficiency of C = 3 - 5 - 4 = -6
Work done by A and B in 12 min = (5 + 4) $$\times$$ 12 = 108 units
Time taken by C to empty the tank = 108/6 = 18 min


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