a cistern can be filled by a tap in 7 hours and emptied by an outlet pipe in 9 hours how long will it take to fill the cistern if both the tap and the pipe are opened together
Answer is 3 hours 15 minutes
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Time taken to fill the tank=7hrs.
Part of tank filled in 1hr=1/7
Time taken to empty the tank(a negative work)=9hours
Part of the tank emptied in 1hr= -1/9
Time taken to fill=1/7-1/9=2/63=63/2=31hours 30mins
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