Frequently Asked Questions about Excel Solver
Below is a partial list of some of the issues that might come up when
you are using Solver. If you find others that you think people should
know about please email sc244@columbia.edu.

Where can I get some instructions on how to use Solver?

There is an article in the readings book (Reading #1, page 11) which gives
a complete workedthrough example.

How do I get Solver working?

No. The only reason you may not see "Solver" in your Tools menu is
that Solver was not loaded when you originally installed Excel. In
that case, go to ToolsAddIns and check off "Solver Add In". Then click
OK and return to the Tools menu. Now Solver should appear.

When do I check off next to "Assume Linear Model" ?

Whenever the model you are solving is a linear model. A model is
linear if decision variables are only multiplied by fixed numbers (constants),
variables are not multiplied together, there are no squares, logs, if statements,
powers etc. used directly in the model (you are allowed to use nonlinear
formulas to calculate parts of your spreadsheet, but they cannot
be part of the constraints or decision variables as specified in Solver).

What does it mean when solver says "The conditions for Assume Linear
Model are not satisfied"?

This means that you have not written a linear model. Somewhere you
have either multiplied decision variables, or used nonlinear expressions
(powers, logs, 1/x). If the model is supposed to be linear, then
you need to fix your model. If the model is supposed to be nonlinear,
then you need to uncheck "Assume Linear Model" in the Solver Options.

What does it mean when solver says "The Set Cells do not converge"?

This means that your problem has an unbounded or infinite solution.
That means that you probably modelled it wrong, and Excel is able to increase
profits to infinity, or decrease cost to negative infinity, or something
like that. To debug this: make sure your constraints are correctly
entered.

What does it mean when I check off "Assume NonNegative" in the Options?

When you do that you are telling the Solver to restrict all the decision
variables (changing cells) to be nonnegative (=positive or 0).
In many decision models, negative decision variables do not make sense
and this is the easiest way to enforce that restriction. (Another
way would be to put in constraints that the decision variables be >=0).

How can I specify several ranges in the "changing cells"?

You can do this by separating them with a comma: e.g. A4:B4,C9:D11.