Two taps can separately fill a cistern 10 minutes and 12 minutes respectively and when the waste pipe is open, they can together fill it in 20 minutes. The waste pipe can empty the full cistern in?
Select
A) 6.5
B) 7.5
C) 8.5
D) 9.5
E) 2
1/10 + 1/12 - 1/x = 1/20   x = 7.5   Answer:7.5
<result>B</result>
Two taps can separately fill a cistern 10 minutes and 12 minutes respectively and when the waste pipe is open, they can together fill it in 20 minutes. The waste pipe can empty the full cistern in?