Step $2$: Add formulas

In cell D$7,$ enter the following formula to calculate the total cost for each plant$-$distributor combination:

Step $3$: Set up constraints

To ensure that each distributor receives at least $80\%$ of their demand, we need to add a constraint. Add the following constraint to the Solver:

Add a constraint that the total units shipped from each plant to each distributor must be greater than or equal to $80\%$ of the forecasted demand.

Step $4$: Set up Solver

In Excel, go to the "Data" tab, click on "Solver" $($in the "Analysis" group$),$ and set up the following parameters:

Set the objective to minimize the total cost $($cell D$11).$

Set the changing variable cells to be the range D$2$:J$5($the unit shipping quantities$).$

Add the constraint described in Step $3.$

Click "Solve" to find the optimal solution.

Explanation:

Once Solver finds the optimal solution, it will adjust the unit shipping quantities to minimize the total cost while satisfying the constraints.

Step $3$

b$.$ The optimal solution will depend on the specific data provided in the table. By solving the model using Solver, you will obtain the optimal unit shipping quantities from each plant to each distributor that minimize the total cost while meeting the demand constraints.

Explanation:

Please note that it is not possible to submit an Excel file directly through this text$-$based interface. However, you can create the spreadsheet and follow the steps outlined above to obtain the optimal solution.