Select multiple items for filter in slicer

Hi! I have a table with the inventory of my products, and also the sales of each product.

In order to know how much inventory I have left before running out, I calculate how much inventory I have this week, and divide it by how many sales in average I had in the past.

So, the formula is:
Inventory / Average weekly sales

The result is a number that tells me how many weeks I still can sell without needing more inventory. If it is too low I have to get more product, if it is too high, I may consider a price drop.

This is my problem:

To calculate the average weekly sales, I use the sales of the last 4 to 20 weeks (the exact number depends on each product). This is no problem when I want to see my result of the current week. But if I need to checks last week´s, or any other week in the past, I have to select that week on the slicer, and manually select the other pasts weeks manually, this has created many trouble and many errors by humans.

What I need is some way to select in one click 4 to 20 weeks, prior to the week from where I need the result.

For example, I have weeks 1 to 20, I want to calculate the average using the last 4 weeks for each week. If I select “week15” on a slicer, I need to also select week14, week13, week12 and week11. Or if I select “week10” the filter should also include “week9, week8, week7 and week6”.

Any help or ideas on how to do this?

Thanks very much