SOLUTION: A water tank has an inlet pipe and a drain pipe. A full tank can be emptied in 30 minutes if the drain is opened and an empty tank can be filled in 45 minutes with the inlet pipe

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Question 210242: A water tank has an inlet pipe and a drain pipe. A full tank can be emptied in 30 minutes if the drain is opened and an empty tank can be filled in 45 minutes with the inlet pipe opened. If both pipes are accidentally opened when the tank is full, then how long will it take to empty the tank?
Answer by Alan3354(69443) About Me  (Show Source):
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A water tank has an inlet pipe and a drain pipe. A full tank can be emptied in 30 minutes if the drain is opened and an empty tank can be filled in 45 minutes with the inlet pipe opened. If both pipes are accidentally opened when the tank is full, then how long will it take to empty the tank?
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The inlet fills 1/45 of the tank per minute.
The drain does 1/30 of the tank per minute.
With both flowing, water (or some liquid) flows in at 1/45 and out at 1/30 per minute.
The net loss per minute is 1/30 - 1/45 = 1/90.
1/90 of the tank per minute --> 90 minutes per tank.