Excel help

Hello,

I have a sales performance sheet where I always update the sale. There is a column for the overall average sales where I need to update it manually as the days go...,
for example if today's date is 22/07/2012, the average column should be updated the average sales automatically. See below:

If the cell "n32" is the sum of the sale of till 22/07/2012, assume 250; then the cell for average "n33" should be = 250/22. And next day when I update it for 23/07/2012, then the average column automatically updated as ( assume the sale is 265, it should be divided by 23!!!

Please help to solve the problem.