A pipe can fill a cistern in 7 minutes whereas the cistern when fill can be emptied by a leak in 14 minutes. When both pipes are opened, find when the cistern will be full?
Options:
(A) 17
(B) 16
(C) 70
(D) 14
(E) 12 Let's think first. Stream of consciousness:
1/7 - 1/14 = 1/14
14 minutes
So, the answer is (D).