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Mixture problem in Calculus

I keep forgetting the way to solve this problem. So, when I in future have no clue I know where to look.

The problem
A 200-galon tank is half full of distilled water. At time t=0, a solution containing 0.5 pound/galon of salt enters the tank at the rate of 5 gal/minutes, and the well-stirred mixture is withdrawn at the rate of 3 gal/minutes.
a. Give a mathematical model representation of the above problem.
b. At what time, will the tank be full?
c. At the time the tank is full, how many pounds of salt will it contain?

The mathematical model
The model describing the above process is based on the following formula:

rate of change of the amount of salt  = (rate of salt arrived) – (rate of salt departed)

Suppose V(t) is the volume of the liquid in the tank at time t. The volume of liquid inside the tank will increase as the inflow (5 gal/minutes) is bigger than the outflow (3 galon/minutes).  Thus, we have

V(t)= 100 galon + (5 gal/min – 3 gal/min) \times t min,
V(t)=(100 + 2t) gal,

as we know that initially the tank is half full (100 gal).

Suppose y(t) is the amount of salt inside the tank at time t. The rate in of salt is as follows,

rate in (pound/min) = (0.5 pound/gal) \times (5 gal/min) = 2.5 pound/min,

while the rate out is
rate out (pound/min) = \frac{y(t)}{V(t)} \times (pound/gal) outflow (gal/min)
= \frac{y}{100+2t} \times 3 (pound/min)
= \frac{3y}{100+2t} (pound/min).

Therefore, the mathematica model is given by,

\frac{dy}{dt}=2.5 - \frac{3y}{100+2t}.          (1)

The solution
First we determine time t at which the tank will be full. We use the following equation,

V(t)=(100 + 2t),

We solve for t when the volume V(t) is 200 galon,  then we obtain t=50.

To solve the differential equation (1) we need the initial condition
as follow,


We are going to use Maple to solve the problem

> model := diff(y(t),t) = 2.5 - (3*y(t))/(100+2*t);
> init := y(0)=0;
> dsolve({model,init});

We obtain the solution that is given by

y(t) = 50+t-\frac{2500 \sqrt{50}}{(50+t)^{3/2}}

When t=50, the amount of salt inside the tank will be

y(50) = 82.32233047.

  1. April 29, 2010 at 3:49 am

    thank you (:

  2. eric (bio 09)
    April 29, 2010 at 12:07 pm

    thank you

  3. eric (bio 09)
    May 5, 2010 at 5:36 am

    pak itu ada yang salah loh kayanya…
    model nya kok :
    model := diff(y(t),t) = 2.5 + (3*y(t))/(100+2*t)
    harusnya kan
    model := diff(y(t),t) = 2.5 – (3*y(t))/(100+2*t)
    pake minus bukan plus…

  4. Ella
    May 5, 2010 at 5:37 am

    pak, model nya salah ya?
    persamaan awal kan 2.5 – rate out trus yg di maple koq 2.5+rate out? yg bener yg mana?

  5. ivanky
    May 5, 2010 at 9:42 am

    Waduh… iya iya bener… sori sori yahhh
    harusnya minusss… saya perbaiki d

    thank you eric dan ella

  6. ivanky
    May 5, 2010 at 9:51 am

    done, it is fixed now.

  1. April 30, 2010 at 7:23 am

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