Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
If Target.Address(0, 0) = "A1" Or Target.Address(0, 0) = "A2" Then
If Range("A1") = "" Or Range("A2") = "" Then
Range("A3") = ""
Application.EnableEvents = True
Exit Sub
End If
Range("A3") = Range("A1") * Range("A2")
ElseIf Target.Address(0, 0) = "A3" Then
Range("A2") = Target / Range("A1")
End If
Application.EnableEvents = True
End Sub