Two taps can separately fill a cistern 5 minutes and 7.5 minutes respectively and when the waste pipe is open, they can together fill it in 18 minutes. The waste pipe can empty the full cistern in?  Choose the correct choice from the following answers:
A) 3.8
B) 3.7
C) 3.6
D) 3.9
E) 3.5
1/5 + 1/7.5 - 1/x = 1/18   x = 3.6   Answer:3.6
<result>C</result>
Two taps can separately fill a cistern 5 minutes and 7.5 minutes respectively and when the waste pipe is open, they can together fill it in 18 minutes. The waste pipe can empty the full cistern in?