Student: A pipe can fill a cistern in 18 minutes whereas the cistern when fill can be emptied by a leak in 20 minutes. When both pipes are opened, find when the cistern will be full?\nOptions:\n(A) 17\n(B) 180\n(C) 70\n(D) 120\n(E) 12.
Another student: Let's say, hmmm...

User
1/18 - 1/20 = 1/180\n180 minutes Final answer: (B).
Assistant