Question

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 ​

Answer 1

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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