At most, k out of n projects will be completed
sum of n projects ≤ k
Project j is selected only if project i is selected
Xj - Xi ≤ 0
Project j must be selected with project i
Xj - Xi = 0
Project j and i is mutually exclusive
Xj + Xi ≤ 1
Project j and i is mutually exclusive but one of them must be selected
Xj + Xi = 1