Question 285929
Let x=amount of time for pool to empty if both pipes are open
Then with both pipes open the pool empties at the rate of 1/x pool per min

Drain pipe empties at the rate of 1/30 pool per min
Inlet pipe fills at the rate of 1/45 pool per min

With both pipes open, the pool empties at the rate of:
1/30-1/45 pool per min, so:
1/30 - 1/45=1/x  multiply each term by 90x
3x-2x=90
x=90 min----- time for pool to empty if both pipes are open

CK 
with inlet pipe open, in  90 min we get 2 pools of water
with drain pipe open, in 90 min we drain 3 pools of water
3 pools - 2 pools= 1 pool of water drained

Hope this helps---ptaylor