Model hs71
  Variables
    x1 = 1, >=1, <=5
    x2 = 5, >=1, <=5
    x3 = 5, >=1, <=5
    x4 = 1, >=1, <=5
    obj = 20
  End Variables

  Equations
    x1*x2*x3*x4 > 25
    x1^2 + x2^2 + x3^2 + x4^2 = 40

    obj = x1*x4*(x1+x2+x3) + x3
  End Equations
End Model