Replace old value with new value in datagrid

programming flash
1

Hello,
My datagrid consist on 4 columns(will focus our attention on 1st and the last column). If the user edit the 1st cell in the “Qty” column (e.g from 1 to 2), as showed in the following images, then the 1st cell of “Pret” column will double as value (e.g from 24.50 to 49).
How do I do that? Here is my code so far

var dp:DataProvider = new DataProvider();
var tempValue:Number;

var col1:DataGridColumn = new DataGridColumn("Qty");
grid.addColumn(col1);
col1.dataField = "Qty";
col1.editable = true;
var col2:DataGridColumn = new DataGridColumn("Denumire");
grid.addColumn(col2);
col2.dataField = "Denumire";
col2.editable = false;
var col3:DataGridColumn = new DataGridColumn("Gramaj");
grid.addColumn(col3);
col3.dataField = "Gramaj";
col3.editable = false;
var col4:DataGridColumn = new DataGridColumn("Pret");
grid.addColumn(col4);
col4.dataField = "Pret";
col4.editable = false;

grid.dataProvider = dp;
grid.editable = true;

var loader:URLLoader = new URLLoader(new URLRequest("meniu.xml"));
loader.addEventListener(Event.COMPLETE, onComplete);
function onComplete(e:Event):void
{
    var xml:XML = new XML(loader.data);
    var itemList:XMLList = xml..item;
    var len:int = itemList.length();
    for (var i:int=0; i < len; i++)
    {
        dp.addItem({Qty:"1",
         Denumire:itemList*.denumire,
         Gramaj:itemList*.gramaj,
         Pret:itemList*.pret});
    }
    grid.addEventListener(DataGridEvent.ITEM_EDIT_END, itemEditPreEnd, false, 100);
    grid.addEventListener(DataGridEvent.ITEM_EDIT_END, itemEditPostEnd, false, -100);
    
    calculateTotal();
}

function itemEditPreEnd(e:DataGridEvent):void
{
    var myGrid:DataGrid = e.target as DataGrid;
    var field:String = e.dataField;
    var row:Number = Number(e.rowIndex);
    if (myGrid != null)
    {
        if (field == "Qty")
        {
            tempValue = myGrid.dataProvider.getItemAt(row)[field];
        }
    }
}

function itemEditPostEnd(e:DataGridEvent):void
{
    var myGrid:DataGrid = e.target as DataGrid;
    var field:String = e.dataField;
    var row:Number = Number(e.rowIndex);
    if (myGrid != null)
    {
        if (field == "Qty")
        {
            var newValue:Number = myGrid.dataProvider.getItemAt(row)[field];
            var prevValue = myGrid.dataProvider.getItemAt(row).Pret;
            if (newValue != tempValue)
            {
                var replacedValue:Number =  prevValue * newValue;
                //trace(replacedValue)
                //code will go here
                calculateTotal();
            }
        }
    }
}

function calculateTotal():void
{
    var result:Number = 0;
    var len:Number = dp.length;
    for (var i:Number = 0; i < len; i++)
    {
        result +=  Number(dp.getItemAt(i).Pret);
    }
    total.text = result.toString();
}