Question

One tap can fill a cistern in 3 hours and the waste pipe can empty the full cistern in 5 hours. In what time will the empty cistern be full, if the tap and the waste pipe are kept open together?

Answer 1

One tap can fill a cistern in = 3 hours

A waste pipe can empty a cistern in = 5 hours

∴ One tap 1-hour work =`1/3`

Waste pipe 1-hour work =`1/5`

∴ One tap and waste pipe together 1-hour work

`=1/3-1/5=(5-3)/15`

`=2/15`

∴ The empty cistern will be full in =`15/2` hours

`=7 1/2` hours