Student: A pipe can fill a cistern in 20 minutes whereas the cistern when fill can be emptied by a leak in 30 minutes. When both pipes are opened, find when the cistern will be full?\nOptions:\n(A) 17 minutes\n(B) 87 minutes\n(C) 60 minutes\n(D) 27 minutes\n(E) 11 minutes.
Another student: Let's say, hmmm...

User
1/20 - 1/30 = 1/60\n60 minutes Final answer: (C).
Assistant