A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?
A)10:00am
B)10:45am
C)12:00pm
D)12:30pm
E)1:30pm
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A tank has both an inlet pipe and an outlet pipe
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Let the tank = 30 liters.guerrero wrote:A tank has both an inlet pipe and an outlet pipe. Working alone, the inlet pipe can fill up the tank in 5 hours. Working alone, the outlet pipe can empty out the tank in 15 hours. If it is desired that the tank should be filled, from empty, exactly 6 hours after the inlet pipe is turned on at 9:30am, then at what time should the outlet pipe be opened ?
A)10:00am
B)10:45am
C)12:00pm
D)12:30pm
E)1:30pm
Rate for the inlet pipe = w/t = 30/5 = 6 liters per hour.
Rate for the outlet pipe = w/t = 30/15 = 2 liters per hour.
When the inlet pipe and the outlet pipe work together, the inlet pipe pumps IN 6 liters per hour, while the outlet pipe pumps OUT 2 liters per hour.
Thus, the NET rate for the 2 pipes working together = 6-2 = 4 liters per hour.
To fill the tank in 6 hours, the required average rate = w/t = 30/6 = 5 liters per hour.
The required average rate (5 liters per hour) is HALFWAY between the rate for the inlet pipe alone (6 liters per hour) and the rate for the 2 pipes working together (4 liters per hour).
Implication:
For 1/2 the time (3 hours) the inlet pipe must work alone.
For 1/2 of the time (the remaining 3 hours) the 2 pipes must work together.
Since the inlet pipe works alone for 3 hours, the outlet pipe is turned on 3 hours after the start time:
9:30am + 3 hours = 12:30pm.
The correct answer is D.
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Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
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