Access:How to calculate cost of good sold

Hello,

I need some basic help .I have a table with three field , Sales Cgs and Average profit. I want to create a form based on that table .the problem is I dont know how to calculate the cost of good sold using access form .I need to calculate cgs and store it in cgs field.

I have to enter cost per item from purchase once a week and enter total sale every day.How do i multipy the quantity of sale with cost per item ?
Thank you.