4. A cistern can be filled by pipe A and B in 12 minutes and 10 minutes respectively. The full tank can be emptied by a third pipe C in 8 minutes only. If all the taps be turned on at the same time, the cistern will be full in how much time?
A.15 1/5 Min B.20 2/5 Min
C.17 1/7 Min D.Cannot be Determined
Answer: 17(1/7) Minutes
