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## Homework Statement

A tank initially contains 400 gal of brine in which 100 lb of salt are dissolved. Pure water is run into the tank at the rate of 20 gal/min, and the mixture (which is kept uniform by stirring) is drained off at the same rate. How many pounds of salt remain in the tank after 30 minutes?

## Homework Equations

## The Attempt at a Solution

These were my variable definitions:

Let y = gallons of brine

y

_{o}=400 gal

d

_{y}/d

_{t}=-20 gal/min

Let w= gallons of water

w

_{o}= 0 gallons

d

_{w}/d

_{t}= +20 gal/min

y=y

_{o}+(d

_{y}/d

_{t})t

y=400gal+(-20gal/min)(30min)

w=w

_{o}+(d

_{w}/d

_{t})(t)

I was concerned because I got a negative answer

The physics side of me says that the water that is being poured in should cause the brine to leave the tank at an even faster rate but that would just give me an even more negative answer.

Thanks