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 o

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Question 303685: 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 opened when the tank is full, then how long will it take to empty the tank
Answer by CharlesG2(834) 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 opened when the tank is full, then how long will it take to empty the tank
1/x * job/min + 1/y job/min = (x + y)/xy job/min
take reciprocal --> xy/(x+y) min/job
x = 30 min, y = 45 min
y is filling and we want emptying so y = -45 min
(30 * -45)/(30 + -45)
-1350/-15
90 minutes to empty tank