1/15 + 1/30 - 1/40 = 9/120
120/9 = 13 3/7
This justifies what answer for what question? Q & A:
Two pipes A and B can fill a cistern in 15 and 30 minutes respectively, and a third pipe C can empty it in 40 minutes. How long will it take to fill the cistern if all the three are opened at the same time?
Options:
(A) 17 1/4 min
(B) 28 1/7 min
(C) 17 1/7 min
(D) 13 3/7 min
(E) 17 5/7 min
(D)