$\"\"$

Let $\"\"$ be the number of pounds of concentrate in the solution at any time $\"\"$ .

The number of gallons of solution in the tank at any time $\"\"$ is $\"\"$ .

The tank loses $\"\"$ gallons of solution per minute.

Therefore, the tank lose concentrate at the rate is $\"\"$ .

The solution gains concentrate at the rate $\"\"$ .

Therefore, the net rate of change is $\"\"$ .

Substitute $\"\"$ .

$\"\"$

$\"\"$ .

$\"\"$

$\"\"$ .