Question

It takes 10 hours to fill a pool with the inlet pipe. it can be emptied in 15 hours with the outlet pipe. if the pool is 2/3 full to begin with, how long will it take to fill it from there if both pipes are open?

Answer 1

Let t be the time the pipes are running. Then:
n/10-n/15=1/3
3n-2n=10
n=10
It will take 10 minutes to fill 1/3 of the pool
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