Consider the capital budgeting problem with seven project represented by binary (0 or 1) variables X₁, X₂, X₃, X₄, X₅, X₆, X₇.

1. Write the constraint modeling the situation in which at most 4 projects from projects 2, 3, 4, 5, 6, and 7 can be selected.
2. Write the constraint modeling the situation in which only 2 of the projects from 2, 3, 4, and 5 must be selected.
3. Write the constraint modeling the situation project 3 or 6 must be selected, but not both.
4. Write the constraint modeling the situation in which at least 3 of the projects from 1, 2, 5, and 7 must be selected.