How to update a MasterClass wch inherts detail class

Question

I have a class RegMaster which has the PackDet as a sub class

In silverlight 4 c# project I binded to the master grid RegMaster - which has master item detail
and has the PackDet (which is binded to Detail grid) the PackDet class is in a common.cs file
which is inherited for RegMaster class


RegMaster Itemsource to the top grid in a screen

RegMaster.PackDet Itemsource to the bottom grid in the same screen

If I change the qunatity in PackDet class, how I could
reflect the change in the master class (i.e RegMaster)


I show the model below here

RegMaster Grid
--------------------------------------------------------------------

CheckBox OrderItemID        ItemIdItemName      ItemQuantity

--------------------------------------------------------------------

Checked        1                  Paracetomol             81
NOTChecked    2                Aspirin               30


--------------------------------------------------------------------



PackDet Grid
--------------------------------------------------------------------

I

BatchNo        Quantity
--------------------------------------------------------------------
#34342        45
#34342        36
--------------------------------------------------------------------
AddButton         RemoveButton



When every add button is clicked PackDet class details should be save to RegMaster class.

Kindly give me the code and support if possible


Thanks for any support
Chocks