Question

A cistern can be filled by pipes a and b in 5 and 8 hours. When full, the cistern can be emptied by pipe c in 10 hours. If all the pipes are turned on at the same time, then the time in which cistern can be filled is

Answer 1

Part filled in 1 hr = (1/5 + 1/8) = 13/40, Part emptied in 1 hr = 1/10

Net filling in 1 hr = (13/40 - 1/10) = 9/40

therefore

Time taken to fill the tank = 40/9 hr

= 4 hours 44 minute