Question 1200985
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The tank can be completely filled in 9 hours.


It takes 4 times as long to empty the tank. 
It takes 4*9 = 36 hours to empty the tank.


Let's say the tank was 36 gallons.


The fill rate is 36/9 = 4 gallons per hour.
Formula:
rate = (amount done)/(time)


The drain rate would be 36/36 = 1 gallon per hour.


Subtract those two rates
4-1 = 3
This indicates the net rate is +3 gallons per hour. 
The plus or positive means more water enters the tank than is drained. 
Overall, the tank is getting filled up even if the drain pipe is open.


If both pipes are open, then it would take 36/3 = 12 hours to fill the tank.
Formula:
time = (amount done)/(rate)


Answer: <font color=red size=4>12 hours</font>
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