Question 628805
A large water tank has two inlet pipes of different sizes and one outlet pipe.
With the outlet pipe closed it takes 3 hours to fill the empty using the large inlet pipe only. It takes 4 hours to fill the empty tank using the small inlet pipe only.
When the tank is full and the inlet pipes are closed it takes the outlet pipe 8 hours to empty the tank.
Assuming the tank is empty and all three pipes are opened how long will it take to fill the tank to the nearest minute. 
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Let x = time (hours) it takes to fill tank
then
x(1/3 + 1/4 - 1/8) = 1
multiplying both sides by 24:
x(8 + 6 - 3) = 24
x(14 - 3) = 24
x(11) = 24
x = 24/11 hours
or
x = 2 hours and 11 minutes