If the following extract from an Excel file for a Linear Programming model of a project network shows the solution using Solver and the representation follows the practice used in class examples, the solution is:

YA

YB

YC

YD

X$1$

X$2$

X$3$

X$4$

X$5$

$1$

$2$

$0$

$2$

$0$

$2$

$4$

$6$

$7$

Sum Product

RHS or constraints

$15$

$10$

$20$

$20$

$0$

$0$

$0$

$0$

$0$

$75$

$1$

$0$

$0\mathrm{.}00$

$1$

$-1$

$1$

$5$

$>=$

$3\mathrm{.}00$

$1$

$-1$

$1$

$6$

$>=$

$6\mathrm{.}00$

$1$

$-1$

$1$

$4$

$>=$

$2\mathrm{.}00$

$-1$

$1$

$2$

$>=$

$0\mathrm{.}00$

A$.$

Crash activity A by $1$ day, activity B by $2$ days and activity C by $2$ days

B$.$

Crash activity A by spending $$15,$ activity B by spending $$10,$ and activity D by spending $$20$

C$.$

Crash activity $2$ by $2$ days, activity $3$ by $4$ days, activity $4$ by $6$ days, and activity $5$ by $7$ days

D$.$

Crash activity A by spending $$15,$ activity B by spending $$20,$ and activity D by spending $$40$