Give a quick stream of consciousness before answering the following question. 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?\nOptions:\n(A) 17 minutes\n(B) 87 minutes\n(C) 70 minutes\n(D) 28 minutes\n(E) 11 minutes
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1/20 - 1/70 = 1/28\n28 minutes
The answer: (D).
Assistant