Bunuel sorry for the stupid question, but after calculating that the drain valve was open for 7.5 hrs, can we not think that maybe it was opened at 2pm and then closed at 9:30pm? (Same logic for Options A, B, C as it will not exceed the 11pm time). Can you please let me know the error in my thought process, thanks.
Bunuel
Official Solution:
A filling pipe can fill an empty swimming pool in 4 hours, while a drain valve can empty the full pool in 5 hours. To fill the empty pool, the filling pipe was opened at 1:00 pm, and after some time, a drain valve was also opened. If the pool was filled at 11:00 pm, when was the drain valve opened?
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 rate of the filling pipe is given as \(\frac{1}{4}\) pool/hour and the rate of drain valve is given as \(\frac{1}{5}\) pool/hour. The filling pipe was opened for 10 hours, from 1:00 pm to 11:00 pm. Let x denote the time for which the drain valve was opened, then we'd have \(10*\frac{1}{4} - x*\frac{1}{5} = 1\), which gives \(x = 7.5\) hours. Hence, the drain valve was opened for 7.5 hours, and therefore was opened at 11:00 pm - 7.5 hours = 3:30 pm.
Answer: D
There’s no indication that the drain valve was ever closed after it was opened. Introducing a closing time adds an extra unknown not supported by the information given, making the problem unsolvable under that assumption. Therefore, the only valid interpretation is that once opened, the drain valve stayed open until the pool was filled.