A pipe can fill a cistern in 20 minutes whereas the cistern when full can be emptied by a leak in 28 minutes. When both pipes are opened, find when the cistern will be full?\nOptions:\n(A) 17\n(B) 16\n(C) 70\n(D) 13\n(E) 12 Now, let's think a bit. Some random thoughts:
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1/20 - 1/28 = 1/70\n70 minutes
The final answer: (C).
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