A cistern can be filled in 9 hours but due to a leak at its bottom it takes 10 hours. If the cistern is full, then the time that the leak will take to make it empty will be ?  Choose one:	A) 20 hours	B) 19 hours	C) 90 hours	D) 80 hours	E) None of these
Explanation:   Part filled without leak in 1 hour = 1/9   Part filled with leak in 1 hour = 1/10   Work done by leak in 1 hour   =1/9−1/10=1/90   We used subtraction as it is getting empty.   So total time to empty the cistern is 90 hours   Option 90 hours
<result>C</result>
A cistern can be filled in 9 hours but due to a leak at its bottom it takes 10 hours. If the cistern is full, then the time that the leak will take to make it empty will be ?