I need to how to calculate avarage purchase prize

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C#

I m using the following code to calculate average purchase prize.I have two classes PurchaseAccountBLL and StockpageBLL. The unit_prize,Quantity, Item_discount is binding from purchase database and Stock_Item_Quantity ,Stock_ItemUnit_Prize,Profit is coming from Stock database table. So I want to calculate the following equation: Profit= (unit_prize*Quantity)-(Stock_Item_Quantity*Stock_ItemUnit_Prize) and update the updated data into stock database table. How I can achieve this.
Thanks in advace

This is my code:


I m retrieve database table data in a list


public List<purchaseaccountbll> PurchaseAccountSelection()
{
DataTable dt = prl.SelectPurchaseAccount();
List<purchaseaccountbll> purchase = new List<purchaseaccountbll>();
foreach (DataRow row in dt.Rows)
{
PurchaseAccountBLL a = new PurchaseAccountBLL();
a.Itemname = row["item_name"].ToString();
a.Id = Convert.ToInt32(row["purchase_id"]);

a.Unit_Prize = Convert.ToDecimal(row["unit_prize"]);
a.Quantity = Convert.ToDecimal(row["quantity"]);
a.Item_Discount = Convert.ToDecimal(row["discount"]);



purchase.Add(a);
}
return purchase;
}


public List<stockpagebll> StockSelection()
{
DataTable dt = prl.StockSelection();
List<stockpagebll> stock = new List<stockpagebll>();
decimal total_prize = 0;
foreach (DataRow row in dt.Rows)
{
StockpageBLL add = new StockpageBLL();
add.Id = Convert.ToInt32(row["item_id"]);
add.Itemname = Convert.ToString(row["itemname"]);
add.Stock_Item_Quantity = Convert.ToDecimal(row["quantity"]);
add.Stock_Barcode = Convert.ToString(row["barcode"]);
add.Stock_ItemUnit_Prize = Convert.ToDecimal(row["unitPrize"]);
add.Profit = Convert.ToDecimal(row["profit"]);

total_prize += Convert.ToDecimal(row["unit_prize"]);
// StockpageDLL stk = new StockpageDLL(Profit,Id);
stock.Add(add);

}
decimal avg = total_prize / dt.Rows.Count;
StockpageDLL stk = new StockpageDLL(avg, barcode);
return stock;


}