Pipe X can fill a tank on its own

Solution

ATQ,

Let the capacity of the tank be 60 litres. {LCM (12, 20 and 15)} Efficiency of pipe 'X' = (60/12) = 5 litres/hour Efficiency of pipe 'Y' = (60/20) = 3 litres/hour Efficiency of outlet pipe 'Z' = (60/15) = 4 litres/hour Quantity of water filled by pipe 'X' and 'Y' together in 4 hours = (5 + 3) × 4 = 32 litres Combined efficiency of all three pipes = 5 + 3 - 4 = 4 litres/hour So, time taken to fill the remaining tank = {(60 - 32)/4} = 7 hours So, total time taken = 7 + 4 = 11 hours