How do I set up a formula to update my inventory after a sale?

Hi everyone thanks in advance for reading

Im trying to figure out a way to input my sales from a drop down menu and then have it automatically update my inventory to reflect those sales, and then have the sales ranked..Is that possible while at the same time adding the cost?

I have attached my files and am referring to my sales page i am trying to focus on the purple parts, and the inventory on the pink.I can delete the date columns part because im not sure that i can make that work..

Or any other suggestions,
Ive been trying but i just dont fully understand the codes and stuff.

Thanks Again,
Lesley

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