Calculating average sales for period

Cannot figure out what might be an easy problem for someone.

I am looking to calculate the average demand for sales of products over a 4 month period.
I have an object with products that has a one-to-many connection to an object with sales. The sales object lists orders by product, customer and date. Sales are uploaded once per month and so the sale dates are always the last calendar day of the month.

How can I combine the product and sales objects with the right formula/equation to show the total average sales for a product over the prior 4 months (I need the total sales, not the average order size)?