A cistern has three pipes, A, B and C. The pipes A and B can fill it in 20 and 15 hours respectively and C can empty it in 30 hours. If the pipes are opened in order at 1, 2 and 3 A.M. When will the cistern be empty? Select:
A) 7 AM B) 5 AM C) 6 AM D) 4 AM E) 11 AM
1 to 2 = 1/20   2 to 3 = 1/20 + 1/15 = 7/60   After 3 AM = 1/20 + 1/15 - 1/30= 5/60   1/20 + 7/60 = 1/6   1 h ---- 5/60   ? ----- 1/6   2hours ==&gt; 5 AM   Answer:5 AM
<result>B</result>
A cistern has three pipes, A, B and C. The pipes A and B can fill it in 20 and 15 hours respectively and C can empty it in 30 hours. If the pipes are opened in order at 1, 2 and 3 A.M. When will the cistern be empty?