## mixing problems

A tank contains 1920 L of pure water. A solution that contains .07 kg of sugar per liter enters tank at the rate 7 L/min. The solution is mixed and drains from the tank at the same rate. With S representing the amount of sugar (in kg) at time t (in minutes) write a differential equation which models this situation.

S'(t)=r1c1-r2c2
r1=r2=7
c1=.07kg
c2=S(t)/V(t)=S/1920

so wouldnt $$S'(t)=.049- \frac{7S}{1920}$$

I'm not sure why this is wrong

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 Recognitions: Gold Member Science Advisor Staff Emeritus Would you believe arithmetic? 0.07*7= 0.49, not 0.049!!