A cistern has two taps which fill it in 12 min. and 15 min.respectively There is one outlet pipe in the cistern. When all the taps & pipe are opened, the empty cistern is full in 20 min. How long will the waste pipe (outlet) take to empty the full cistern ?
A. 10 min
B. 20 min
C. 30 min
D. 40 min
Work done by waste pipe in 1 minute
=1/20−(1/12+1/15)=−1/10
Waste pipe will empty the full cistern in 10 minutes.