Let XA and XB be decision variables corresponding to taking up project A and project B (eg: XA =0 if don't take up A and XA = 1 if take up A).

Question: How to model (write out an inequality or equation that conveys the logic below):

1. Can undertake project A or B but not both

2. Can undertake A only if B is also done

3. Can undertake A and B only if done together

4. Have to undertake A if B is taken up for investment