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 9:00 pm and not earlier, when was the drain opened?
Choose one
A) at 2:00 pm	B) at 2:30 pm	C) at 3:00 pm	D) at 3:30 pm	E) at 4:00 pm
The filling rate is 1/4 and the draining rate is 1/5.   The combined rate of filling and draining is 1/4 - 1/5 = 1/20.   Let x be the number of hours before the drain was opened.   x/4 + (8-x)/20 = 1   5x + 8 - x = 20   4x = 12   x = 3   The answer is 4:00 pm.   The answer is at 4:00 pm.
<result>E</result>
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 9:00 pm and not earlier, when was the drain opened?