Solve Mixing Problems: Differential Equation Modeling

[UrbanXrisis]

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 wouldn't S'(t)=.049- \frac{7S}{1920}

I'm not sure why this is wrong

 

[HallsofIvy]

Would you believe arithmetic? 0.07*7= 0.49, not 0.049!