2 pipes A and B can separately fill a cistern in 10 and 15 mins respectively. A person opens both the pipes together when the cistern should have beenwas full he finds the waste pipe open. He then closes the waste pipe and in another 4mins the cistern was full. In what time can the waste pipe empty the cistern when fill?  Select the correct option:
A) 6
B) 8
C) 9
D) 10
E) 12
1/10 + 1/15 = 1/6 * 4 = 2/3   1 - 2/3 = 1/3   1/10 + 1/15 - 1/x = 1/3   x = 8   8
<result>B</result>
2 pipes A and B can separately fill a cistern in 10 and 15 mins respectively. A person opens both the pipes together when the cistern should have beenwas full he finds the waste pipe open. He then closes the waste pipe and in another 4mins the cistern was full. In what time can the waste pipe empty the cistern when fill?