1/20 - 1/30 = 1/60
60 minutes
A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 30 minutes. When both pipes are opened, find when the cistern will be full?
a: 17 minutes
b: 87 minutes
c: 60 minutes
d: 27 minutes
e: 11 minutes