Q: 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) 22 minutes
(B) 88 minutes
(C) 70 minutes
(D) 77 minutes
(E) 48 minutes Let's give some random thoughts before answering.

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
1/20 - 1/28 = 1/70
70 minutes
Therefore, the answer is (C).