This topic has expert replies
Master | Next Rank: 500 Posts
Posts: 228
Joined: 17 Aug 2008
Thanked: 4 times

Work Rate

by jainrahul1985 » Sat Jul 25, 2009 6:20 am
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

User avatar
Legendary Member
Posts: 519
Joined: 27 Jan 2007
Location: India
Thanked: 31 times

by Neo2000 » Sat Jul 25, 2009 8:34 am
Though a time and work question, this can easily be solved without applying any formula.

The tap that fills was opened at 1pm and at 11pm the tank was full => the tap was open for 10hrs
Given the rate of filling, we know that 2.5times the tank capacity of water was poured into the tank, but tank only got full at 11pm. which means 1.5times the tank capacity drained away.

The drain tap can drain the capacity tank in 5hrs. How many hours will it take to drain 1.5times the tank capacity = 7.5hrs

Therefore if the drain tap was open for 7.5hrs ending at 11pm, it was initially opened at 3:30pm.

Mentally, this should take you abt 20secs.