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?
Options:
(A) 17
(B) 180
(C) 70
(D) 120
(E) 12 Now, let's think a bit. Some random thoughts:
1/18 - 1/20 = 1/180
180 minutes
The final answer: (B).