Allocation problem for optimize grade with steps between in Excel

Hello,
I am a beginner in Excel programming. I have to following allocation problem.

There are lectures which can be assigned to one or more of a total of four categories. Each lecture has a grade and contributes two to four credit points to the category assigned. The given credit points are independant from the assigned category.
The categories have an grade which results from the unweighted average of the assigned lectures. The total grade results from the unweighted average of the categories. The total grade is to minimized under the restriction that every category has at least 10 credit points.

I know how to solve a simple transportation problem with solver but the major problem in this task is that lecture are assigned to a category during the optimization process. Depending on which lectures are assigned to which category the average grade of the category changes. I do not not how to compute this steps in Excel.

Thanks in advance for your help.

Homework?

Forum rules says

In this forum we cannot allow direct help with homework or assignment questions

But try uploading a workbook showing your problem and what you have done so far. Hopefully then somebody may give you a hint or two helping you to solve your problem.

Alf