Comparing data of current week and last week while calculating sell-through

[Anonymous]

Hi all! I am calculating the sell through rate of a product.

Right now, I have weekly sales and inventory data.

The formula of the rate is the sales quantity divided by the inventory of the last week.

However, I don't know how to write the expression of the measures.

I have tried the following but it won't work:

SUM(Sales)/SUM({<[Date]={[Date]}-7>}Inventory)

Please help solve! Thanks in advance!!

p.s. This is the expected answer:    

Date	Sales	Inventory	Sell Thru
7/1/2017	6	54
7/8/2017	1	21	2%
7/15/2017	3	53	14%
7/22/2017	6	21	11%
7/29/2017	4	66	19%
8/5/2017	7	53	11%