Closing stock calculation

I have received accounts of one propriotory concern who deals in paper bags and stationary.  Earlier accountant was not following the method of unit wise entry into tally package.  Now my problem is how to find out Closing Stock every month.  (Earlier Accountant used to calculate on lumpsum basis).  Please guide me as to which method to use to calculate closing stock when purchase, sales and opening stock is given (all including GST)  

suppose his sales for the month is 125143, purchase is 29778 and opening stock is 439980.  CA has shown 363500 as closing stock for the month.  What calculation has been done.  Please guide me.