Weighted average based on Days

I am trying to create a weighted Average based on days.

For Sales.

Day 1 $100

Day 2 $200

Day 3 $150

...

Day 31 $175

These sales contribute to assets.   Essentially day 1 sales have been "contributing" for 31 days, day 2 sales 30 days etc.

The Weighted average should be:

Day 1 sales*(31/31) + Day 2 sales*(30/3) + day 3 sales* (29/31)....+day 31 sales *(1/31)

I need to also be able to break this weighted average down by other dimensions such as fund.

Any suggestions on how to accomplish this in a BI query without having to do work a BI workbook?

Jodi