SOLUTION: 2 pipes are connected to a tank. On pipe fills the tank in 7 hours. The second pipe empties the tank in 9 hours. If both pipes are open, how long will it take to fill the tank?

Algebra ->  Test -> SOLUTION: 2 pipes are connected to a tank. On pipe fills the tank in 7 hours. The second pipe empties the tank in 9 hours. If both pipes are open, how long will it take to fill the tank?      Log On


   



Question 380404: 2 pipes are connected to a tank. On pipe fills the tank in 7 hours. The second pipe empties the tank in 9 hours. If both pipes are open, how long will it take to fill the tank?
Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
Find the hourly rate of each event.
If the first pipe can fill the tank in 7 hours, then it can fill (+) 1%2F7 of the tank in 1 hour.
If the second pipe can empty the tank in 9 hours, then it can empty (-) 1%2F9 of the tank in 1 hour.
Putting these two events together, we get:
1%2F7+-+1%2F9+=+2%2F63
So 2%2F63 of the tank is filled in 1 hour.
It takes 63%2F2 hours to fill the tank.
63%2F2+=+311%2F2hours.