Formulas to calculate multiple data validation responses

Interesting Problem.

Lets start with Multiple Selection Validation for Column C. This is possible using a simple Macro.

Private Sub Worksheet_Change(ByVal Target As Range)
Dim Oldvalue As String
Dim Newvalue As String

On Error GoTo Exitsub
If Target.Address <> "$A$1" Or Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Or Target.Value = "" Then GoTo Exitsub

Application.EnableEvents = False

N = Target.Value
Application.Undo

If Target.Value = N Then Target.Value = "": GoTo Exitsub

If Target.Value <> "" Then O = Target.Value & ", "

If InStr(O, N & ", ") = 0 Then
T = O & N & ", "
Else:
T = Replace(O, N & ", ", "")

End If
 
T = Left(T, Len(T) - 2)

Target.Value = T

Exitsub:
Application.EnableEvents = True
End Sub

Ok I have modified that macro for your requirement.

Modifying A3 0r A8 will change the Data Validation in C3 and C8.

You can then make multiple selections in C3 and C8

Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Cells.Count > 1 Then Exit Sub

If Not Intersect(Target, Range("A3:A8")) Is Nothing Then

     Ans = "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"
     
     DataT = Left(Ans, InStr(Ans, Target.Value) + Len(Target.Value) - 1)
     
         With Cells(Target.Row, 3).Validation
        .Delete
        .Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
        xlBetween, Formula1:=DataT
        .IgnoreBlank = True
        .InCellDropdown = True
        .InputTitle = ""
        .ErrorTitle = ""
        .InputMessage = ""
        .ErrorMessage = ""
        .ShowInput = True
        .ShowError = True
    End With

End If

If Not Intersect(Target, Range("C3:C8")) Is Nothing Then
Application.EnableEvents = False

N = Target.Value
Application.Undo

If Target.Value <> "" Then O = Target.Value & ", "

If InStr(O, N & ", ") = 0 Then
T = O & N & ", "
Else:
T = Replace(O, N & ", ", "")

End If
 
T = Left(T, Len(T) - 2)

Target.Value = T

Exitsub:
Application.EnableEvents = True
End If

End Sub

You would need to explain the next stage, clearly with examples.