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Mixture problem (with a twist) Diff. Eq.

  1. Sep 19, 2008 #1
    1. Beginning at time t=0, fresh water is pumped at the rate of 3 gal/min into a 60-gal tank initially filled with brine. The resulting less-and-less salty mixture overflows at the same rate into a second 60-gal tank that initially contained only pure water, and from there it eventually spills onto the ground. Assuming perfect mixing in both tanks, when will the water in the second tank tast the saltiest. Exactly how salty will it be at this time compared to the original brine?

    2. The obvious mixture form.... (dA/dt)= (Ri-Ro), but I can't get my brain wrapped around this to start setting it up.

    3. The simple mixture problem, with an amt. of liquid * mass of substance being incorporated into a already mixed fluid with a certain concentration, with a rate of change, is simple to set up....but I don't know exactly where to start.
  2. jcsd
  3. Sep 19, 2008 #2


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    First solve the problem of finding the concentration in the first tank as a function of time. That gives you the Ri for the second tank. Now it's just the usual problem.
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