wo pipes X and Y fill a tank in 15 hrs. and 20 hrs. respectively, while a third pipe ‘Z’ can empty the full tank in 25 hrs.  All the three pipes are opened in the beginning.  After 10 hrs. Z is closed.  In how much time, will the tank be full ?

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wo pipes X and Y fill a tank in 15 hrs. and 20 hrs. respectively, while a third pipe ‘Z’ can empty the full tank in 25 hrs.  All the three pipes are opened in the beginning.  After 10 hrs. Z is closed.  In how much time, will the tank be full ?

A. 12 hrs
B. 14 hrs
C. 16 hrs
D. 18 hrs
Explanation:
Part filled in 10 hrs.
= 10[1/15+1/20−1/25]=23/30