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? Choose the correct choice from the following answers.
A) 6 PM
B) 7 PM
C) 8 PM
D) 5 PM
E) 9 PM
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 ==&gt; 5 PM   Answer:5 PM
<result>D</result>
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?