Student: A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 28 minutes. When both pipes are opened, find when the cistern will be full?
Options:
(A) 17 minutes
(B) 87 minutes
(C) 70 minutes
(D) 27 minutes
(E) 11 minutes.
Another student: Let's say, hmmm...
1/20 - 1/28 = 1/70
70 minutes Final answer: (C).