Pipe ‘A’ and pipe ‘B’ can fill a cistern in 20 minutes and 15 minutes respectively. Pipe ‘C’ alone can empty the cistern in 10 minutes. If all three pipes are opened together then what is the time taken to fill 25% of the cistern?
Let the capacity of the cistern = 60 units Then, efficiency of pipe ‘A’ = 60/20 = 3 units/minute Efficiency of pipe ‘B’ = 60/15 = 4 units/minute Efficiency of pipe ‘C’ = 60/10 = 6 units/minute So, combined efficiency of pipes ‘A’, ‘B’ and ‘C’ = 3 + 4 – 6 = 1 units/minute 25% of the cistern’s capacity = 15 units Therefore, time taken by all 3 pipes together to fill 25% of the cistern = 15/1 = 15 minutes
