# Please check my work below and comment.

A tank initially contains 80 gallons of fresh water. A 10% acid solution flows into the tank at the rate of 3 gallons per minute. The well-stirred mixture flows out of the tank at the rate of 3 gallons per minute. Find the amount of acid in the tank at the end of any time t. How much acid will be in the tank in 30 minutes? What will be the concentration (%) of acid in the tank after 30 minutes?

solution=0.3 gallons acid + 2.7 gallons H20

dy/dt = (rate in) - (rate out)
rate in = (0.3)(3) = 0.9 gallons/min
<-would this be gallons per min?

rate out = ((y(t))/80) * 3 gal/min
rate out = y(t)/(80/3)

dy/dt=(0.9)-(y(t))/(80/3)
dy/dy=((773/30)-y(t))/(80/3)

Separate into two intergrals:
Integral(dy/((773/30)-y)=Integral(dt/(80/3))
-ln|(773/30)-y| = 3t/80+C, C=-ln|773/30|
-ln|(773/30)-y|=3t/80 - ln|773/30|
y(t)=(773/30)-(773/30)e^(-3t/80)

Is this the correct equation?

Then at 30 minutes, y(30) would be 17.4 gallons acid

Percentage after 30 minutes=17.4/80
=21.8% acid???

Thanks.

correct procedure, however, one question: how did you get rate in .3*3? Wasn't it ten percent?

ah yes. Thank you!

## You're welcome! I see that there was a mistake in your calculation for the rate in. The concentration of acid in the solution flowing into the tank is indeed 10%. So, the correct calculation should be 0.1 * 3 = 0.3 gallons per minute for the rate in.

Now, let's go through the calculations again starting with the correct rate in:

dy/dt = (rate in) - (rate out)
rate in = 0.1 * 3 = 0.3 gallons per minute

rate out = (y(t) / 80) * 3 gallons per minute

dy/dt = 0.3 - (y(t) / (80/3))

To solve this differential equation, you can separate variables and integrate:

∫(1 / (0.3 - y)) dy = ∫dt / (80/3)

Integrating both sides:

-ln|0.3 - y| = (3t) / 80 + C

To find the value of C, we can substitute y(0) = 0 for the initial condition:

-ln|0.3 - 0| = (3 * 0) / 80 + C
C = -ln(0.3)

Now, substitute the value of C back into the equation:

-ln|0.3 - y| = (3t) / 80 - ln(0.3)

To solve for y(t), we can exponentiate both sides:

|0.3 - y| = e^((3t) / 80 - ln(0.3))

Considering both possible cases:

0.3 - y = e^((3t) / 80 - ln(0.3)) (1)
or
y - 0.3 = e^((3t) / 80 - ln(0.3)) (2)

To determine which equation to use, you can substitute t = 0 to find the initial condition:

In equation (1):
0.3 - y(0) = e^(-ln(0.3))
0.3 - y(0) = e^(ln(1/0.3))
0.3 - y(0) = 1/0.3
0.3 - y(0) = 10/3

In equation (2):
y(0) - 0.3 = e^(-ln(0.3))
y(0) - 0.3 = e^(ln(1/0.3))
y(0) - 0.3 = 1/0.3
y(0) - 0.3 = 10/3

From both cases, we can observe that y(0) = 10/3 - 0.3 = 10/3 - 9/30 = 1/15

Thus, we use equation (2) as it satisfies the initial condition:

y - 0.3 = e^((3t) / 80 - ln(0.3))

Now let's continue with the calculations using the correct equation:

y = 0.3 + e^((3t) / 80 - ln(0.3))

After substituting t = 30 minutes:

y(30) = 0.3 + e^((3 * 30) / 80 - ln(0.3))
y(30) = 0.3 + e^(90/80 - ln(0.3))
y(30) ≈ 17.43 gallons of acid

Finally, to find the percentage of acid in the tank after 30 minutes:

Percentage after 30 minutes = (17.43 / 80) * 100
Percentage after 30 minutes ≈ 21.79%

Therefore, the amount of acid in the tank after 30 minutes would be approximately 17.43 gallons, and the concentration of acid in the tank would be approximately 21.79%.