Question 276432
if it takes 10 hours to fill a pool and 20 hours to drain it, how long will it take to fill it if the drain is open?

It fills 1/10th. of the pool in one hour

It drains 1/20th of the pool in 1 hour.

In an hour actually the increase is 1/10-1/20
1/20

with the drain open 1/20th of the pool is filled in one hour.

so to fill the pool it will take 20 hours