1/20 - 1/70 = 1/28
28 minutes
A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 70 minutes. When both pipes are opened, find when the cistern will be full?
[A] 17 minutes
[B] 87 minutes
[C] 70 minutes
[D] 28 minutes
[E] 11 minutes