x1+x3+x4+x5 b. Write a constraint modeling a situation in which, if projects 3 and 5 must be undertaken, they must be undertaken simultaneously. For those boxes in which you must enter subtractive or negative numbers use a minus sign. (Example: 300 ) c. Write a constraint modeling a situation in which project 1 or 6 must be undertaken, but not both. For those boxes in which you must enter subtractive or negative numbers use a minus sign. (Example: 300 ) x1+x6 d. Write constraints modeling a situation where project 6 cannot be undertaken unless projects 1 and 3 also are undertaken. For those boxes in which you must enter subtractive or negative numbers use a minus sign. (Example: 300 ) \begin{tabular}{l|l} x6 & x1 \\ x6 & x3 \end{tabular} e. Write constraints modeling a situation where project 6 cannot be undertaken unless projects 1 and 3 also are undertaken. Assume that when projects 1 and 3 are undertaken, project 6 also must be undertaken. For those boxes in which you must enter subtractive or negative numbers use a minus sign. (Example: 300 ) e. Write constraints modeling a situation where project 6 cannot be undertaken unless projects 1 and 3 also are undertaken. Assume that when projects 1 and 3 are undertaken, project 6 also must be undertaken. For those boxes in which you must enter subtractive or negative numbers use a minus sign. (Example: 300 )