Question 398759
An empty swimming pool can be filled in 10 hours. When full. the pool can be drained in 19 hours. How long will it take to full the empty pool if drain is left open.

pipe A ----- 10 hours filling
so it does 1/10 of job in 1 hour,
Pipe B ----------- 19 hours draining
so it does 1/19 of the job in 1 hour.
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1/10 -1/19 is the job done in 1 hour when both are open
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(19-10)/190= 9/190 of the job 
..
so it will take 190/9 hours to fill the pool.
=21.11 hours