I am not sure why you have a grouping on the items using 100, 200, 300 etc. You should have the items as belonging to a category. If the items are also part of a number grouping, is this within a category, or across all items. I would need to see a schema of this database.
But basically here is how you do this:
I am assuming you have a table called tblCategory and one called tblItems that look like this:
CatID - PK
CatName - text
ItemID - PK
ItenName - text
CatID - FK
On ONE unbound form (myForm), you have your combo box to select the Category (cboCat), and your listbox with Items (lstItems).
The cboCat is just a wizard that looks up the CatName and binds on the CatID.
The row source for lstItems is a query of the items that looks like this:
SELECT tblItem.ItemID, tblItem.ItemName, tblItem.CatID
ORDER BY tblItem.ItemName;
The AfterUpdate event for cboCat is:
Private Sub cboCat_AfterUpdate()
Me.lstItem = Null
Me.lstItem = Me.lstItem.ItemData(0)
The On Current event for the form is:
Private Sub myForm_Current()
The On Load event for the form is:
Private Sub myForm_Load()
Me.cboCat = Me.cboCat.ItemData(0)
This should work. I really need to see more of the schema.