Question

a cistern has two pipes attached to it.one pipe can supply 3/4 L and the other can draw off 4/5 L of water in a minute .if both the pipes are opened at the same time when the cistern has 27 litres of water,how long will it take to empty the cistern?

Answer 1

3/4-4/5= 16-15/20
it will take 20 minute to empty