Part of a summation

Hello @Lau, your question is not clear. 

If x1, x2, … x5 are sets, then how do you want to apply a constraint on sets ?

I suppose you mean x1, x2 .. x5 are elements in your set with index i, and you have a variable - say abc(i), and the constraint is 

abc(x5) = 10*abc(x1) + 18*abc(x2) 

You can write this constraint pretty much like above, just enclose the element names in single quotes. 

abc(‘x5’) = 10*abc(‘x1’) + 18*abc(‘x2’) 

Is this correct ? If not, can you clarify ?