Formula that calculates cumulative averages

[lonniepoet]

In the attached spreadsheet on the 2016 tab I need a formula in column J to take the average of the cell on the left for column I plus all the cells above it and give me an average. For instance, I need cell J3 to give me an average of cells I2 & I3. I would then need the next cell for J4 to give me an average for I2, I3 & I4 and so on. I can’t figure out how to make it do the cell to the left plus all the cells before it. Any help would be much appreciated. I would also like to know how/why a solution works so that I can figure it out for future use. Thank you in advance!