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

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Question 353688: a water tank has an inlet pipe and a drain pipe. a full tank can be emptied in 30 min if the drain is opened and an empty tank can be filled in 45 min 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 nerdybill(7384) 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 min if the drain is opened and an empty tank can be filled in 45 min 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?
.
Let x = time (minutes) to empty tank
then
x(1/30 - 1/45) = 1
Multiplying both sides by (30)(45):
x(45 - 30) = (30)(45)
x(15) = 1350
x = 90 minutes