Summing up quantity of product sold in Purchase order form and displaying the same number in the Products form

Summing up quantity of product sold in Purchase order form and displaying the same number in the Products form

Hello All,

Actually i have a main form-Purchase Order and a subform in it-Purchase_subform which includes Products,Price,Quantity and Total field. Now i am entering the products in the subform alongwith the price and quantity for multiple products. I want to calculate the total quantity of each and every product sold in the subform in this month for all the Purchase Order records of this month. I need to store it in a field 'No of units sold' in the Products form.

I am unable to figure out the solution. I tried multiple ways but cannot find a solution. Can someone please help me on this.