Visual Studio 2017 - Weighted average based on selection

Dear Gurus,
I am developing an application connected to an ACCESS DB.
In the DB I have a table where I store 31 votes per each row. Now, I need to create a datatable where I will add a column in which I would like to calculate the weighted average of the 31 votes. The tricky part is that I want the user to select 4 different votes to which assign the weights (e.g. 5 to the first one, 4 to the second one, 3 to the third one, 2 to the fourth one and 1 to all the other).
I thought to allow the random selection with four comboboxes where the user will select the four votes. Then the code should apply the formula and store the calculated value in the last column of the datatable per each row.

Any clue on how I could achieve it?

Thanks,
A.