A cistern has two taps which fill it in 12 minutes and 15 minutes respectively. There is also a waste pipe in the cistern. When all the three are opened, the empty cistern is full in 20 minutes. How long will the waste pipe take to empty the full cistern ?
Choose the correct answer:
A) 10 min
B) 12 min
C) 15 min
D) 9 min
E) None of these
Work done by the waste pipe in 1 minutes   = 1⁄20 - (1⁄12 + 1⁄15) = -1⁄10 [-ve sign means emptying]   ∴ Waste pipe will empty the full cistern in 10 minutes.   10 minnswer 10 min
<result>A</result>
A cistern has two taps which fill it in 12 minutes and 15 minutes respectively. There is also a waste pipe in the cistern. When all the three are opened, the empty cistern is full in 20 minutes. How long will the waste pipe take to empty the full cistern ?