I would like to define the inlet of a chemical unit but when I try to define an equation for the flow, there is an error saying that

"Too many equations, over-determined system. The model has 5 equation(s) and 4 variable(s)."

Here is the test :

package Test9

constant Integer nbr = 3 ;

connector Mixture
  Real C[nbr];
  flow Real q_aq ;
end Mixture;

 
class inlet
    parameter Real Q = 5.55;
    parameter Real C1 = 2.3;
    parameter Real C2 = 0.12;
    parameter Real C3 = 0.475;
    Mixture mixture;
  equation
    mixture.q_aq = Q*2.5 ;
    mixture.C[1] = C1;
    mixture.C[2] = C2 * 2.;
    mixture.C[3] = C3 * 3.1245;
  end inlet;
 
end Test9;

Thank for your help