Two taps can separately fill a cistern 20 minutes and 30 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 option
A) 38
B) 37
C) 36
D) 39
E) 35
1/20 + 1/30 - 1/x = 1/18   x = 36   Answer:36
<result>C</result>
Two taps can separately fill a cistern 20 minutes and 30 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?