Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Count > 1 Or Target.Column <> 2 Then Exit Sub
Dim Est As Range, c As Range
For Each c In Range("C1:BT1")
Set Est = Range("A2:A21").Find(c, LookIn:=xlValues, LookAt:=xlWhole)
If Est Is Nothing Then Cells(Target.Row, c.Column) = c.Value
Next
End Sub