A cistern has three pipes, A, B and C. The pipes A and B can fill it in 5 and 6hours 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:
A) 8hrs 25min. (AM)	B) 8hrs 54min. (AM)	C) 7hrs 45min. (AM)	D) 7hrs 15 min. (AM)	E) 7hrs 30min. (AM)
1 to 2 = 1/5   2 to 3 = 1/5 + 1/6 = 11/30   After 3 AM = 1/5 + 1/6 - 1/2= -4/30   1/5 + 11/30 = 17/30   1 h ---- 4/30   ? ----- 17/30   7hrs 15 min. (AM)   Answer:7hrs 15 min. (AM)
<result>D</result>
A cistern has three pipes, A, B and C. The pipes A and B can fill it in 5 and 6hours 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?