help to get sum on the field

honestly im still in the process of planning my database so i haven't made the entire flow and practicing how to build from forms

but the idea i have is this:

i would like to have a form for purchase items for my inventory stocks, second is a form for sales that i have sold/ deployed and a form for reports.

Purchase items form would contain:

*date of purchase
* supplier
* Item name
* total quantity of my purchase

Sold/ Deployed Form items form would contain:

* date of sales
* client name
* item name
* total quantity of sales

Report form/ inventory contains the following format:

i.e.

Item Name            Total Quantity of my purchase            Total Items Sold                      Total quantity that i need to purchase
                                     (from purchase form)                    (Sold/ Deployed Form)                  (calculation)
===================================================================================================

Toy #1                                 35                                                       20                                                   20 (formula below)

                                                                                                                                               = if total quantity from Total Items Sold is
                                                                                                                                                                                                => 20, then show quantity of 100

thanks and hope you can show me how would this work before i start to build up my own . Thank you in advance