Average Number of Days in a Quarter

I have a worksheet containing 5 columns. A start and end date, a number of days between these dates, the quarter number of the start date (i.e. 1 to 4) and then the year.

I want to calculate the average number of days per quarter so an average of days in column C, based on the details in column D and E.

Can anyone help?

I have attached an example workbook.

Thanks
dvent