Question 566089
Let x=time required for both drains working together to drain the pool
Then both drains working together drains at the rate of 1/x pool per hour
One drain drains at the rate of 1/20 pool per hour
Another drains at the rate of 1/25 pool per hour

Together they drain at the rate of 1/20 + 1/25 pool per hour, soooo

1/20+1/25=1/x  multiply each term by 100x
5x+4x=100
9x=100
x=11.11 hours

CK
In 11.11 hours one drains drains 11.11/20 of the pool
The other drains 11.11/25 of the pool
11.11/20 + 11.11/25 needs to equal 1 (1 pool, that is)
55.55/100 + 44.44/100=99.99/100  Close enough

Hope this helps---ptaylor