Question

A pipe can fill a cistern in 12 minutes and another pipe in 15 minutes but a third pipe can empty in 6 minutes.The first two pipes are kept open for 5 minutes in the beginning and then the 3 rd pipe is also opened .In what time will the cisterned be emptied?

Answer 1

filled cistern by first two pipes in 5m=5/12×2=5/6 part of cistern
& emptied part of cistern by third pipe in 5m=5/15=1/3 part of cistern
so taken time by 3rd pipe for emptied cistern=5/6-1/3=3/6=1/2 so cistern filled 1/2 part in 5m so it will be full fill in 10m
so emptied time will be also 5minutes