Enter "x" into a cell that represents a number/score then produce an average score.

Hi there,
I'm currently trying to format a worksheet to be used as the company appraisal form. For this I have many categories that the employee will be scored in. They will be scored 1-5 & N/A if the category is not relevant to their position. If a "x" is placed in a box, it represents the value/score of the column then the average score is calculated for that section of questions. If a "x" is entered in the N/A column, that question is exempt from the average score calc. I'm no wiz with excel and this has proven to be above my ability. Any help with this would be great as my boss is desperate for this form to be returned working correctly.

Thanks

Fraser.