Please Help Average Formula Support

One way to accomplish this is with a site workflow.  I am assuming a couple of things: the Score column is a choice field with 3 choices (100, 0, N/A) and each item in the list has only one score.  Here are the steps:

1. Create a site workflow.
2. Add a "Query List" action.  Configure it with a Filter to select all items when Score not equals "N/A" and store the ID fields in a Collection workflow variable (for example, colScores).
3. Add a "Collection Operation" action.  Configure it with the Target Collection from Step 2, select Count option and "Store Result In" a Number workflow variable (for example, numScoresCount).
4. Add a "For Each" action to loop through the colScores collection and store the result in a Text workflow variable (for example, txtID).
5. Add a "Set Variable" action within the "For Each" action.  Configure that action to set a Number workflow variable (for example, numSingleScore) to look up the Score value for each item in the collection and store the choice (100 or 0).
6. Add a "Math Operation" within the "For Each" action to add the numSingleScore variable to a total sum number workflow variable (for example, numScoresSum) and store the result in the numScoresSum variable.
7. Add a "Math Operation" outside the "For Each" action.  Configure it as numScoresSum divided by numScoresCount and store result (average) in a Number workflow variable (for example, numScoresAvg).
8. That gives you the average score of all Scores in a list that are not "N/A".

Below is an image of the workflow design: