Question

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

Answer 1

In 1 hour the 1st pipe or the filling pipe can fill the cistern by 1/3 part.

In 1 hour the empty pipe can empty the cistern by 1/5 part.

So, In 1 hour the cistern can be filled by 1/3-1/5 = (5-3)/15 = 2/15 part.

So, the cistern will be fully filled in 15/2 = 7 hours and 30 minutes.