Solve variable equation system in Simulink

Hello,

I have the following equation system:

Y = X1/3*A+X1/3*B+X1/3*C + X2/3*D+X2/3*E+X2/3*F
X1+X2=X

X,Y,A,B,C,D,E,F are known.

Usually I can solve this in simulink by using matrix form A*X=b quite easy. But my 6 terms which are normally equal X1/3 or X2/3 are limited. If one of the term X1/3 lies above the limit I have to restrict the value to the limit and do the calculation again to fulfill Y. What is the best way to solve this problem in Simulink?