A cistern has three pipes, A, B and C. The pipes A and B can fill it in 4 and 5 hours respectively and C can empty it in 2 hours. If the pipes are opened in order at 1, 2 and 3 A.M. When will the cistern be empty?

A. 3 PM
B. 7 PM
C. 4 PM
D. 5 PM

Explanation:

1 to 2 = 1/4

2 to 3 = 1/4 + 1/5 = 9/20

After 3 AM = 1/4 + 1/5 – 1/2 = -1/20

1/4 + 9/20 = 14/20

1 h —- 1/20

? —– 14/20

14 hours ==> 5 PM