A pool can be filled in 4 hours and drained in 5 hours. The valve that fills the pool was opened at 1:00 pm and some time later the drain that empties the pool was also opened. If the pool was filled by 11:00 pm and not earlier, when was the drain opened?
* at 2:00 pm
* at 2:30 pm
* at 3:00 pm
* at 3:30 pm
* at 4:00 pm









t/4 + (10-t)/20 = 1
5t + 10 - t = 20
4t = 10
t = 2.5
So the drain was turned on 2.5 hours after 1pm, or at 3:30pm