Two taps can separately fill a cistern 5 minutes and 10 minutes respectively and when the waste pipe is open, they can together fill it in 15 minutes. The waste pipe can empty the full cistern in?
Choose the correct option.
A) 7
B) 8
C) 6
D) 4.2
E) 5
1/5 + 1/10 - 1/x = 1/15   x = 30/7 = 4.2   Answer: 4.2
<result>D</result>
Two taps can separately fill a cistern 5 minutes and 10 minutes respectively and when the waste pipe is open, they can together fill it in 15 minutes. The waste pipe can empty the full cistern in?