A pipe can fill a tank in 9 hours. Due to a leak in the bottom,it is filled in 10 hours. If the tank is full. then the time (in hours) that the leak takes to empty that tank is
Part filled without leak in 1 hour $$= \dfrac{1}{9}$$
Part filled with leak in 1 hour $$=\dfrac{1}{10}$$
Work done by leak in 1 hour$$=\dfrac {1}{9}-\dfrac{1}{10}=\dfrac{1}{90}$$
We used subtraction as it is getting empty.
So total time to empty the cistern is 90 hours
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