Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column <> 2 Or Target.Count <> 1 Then Exit Sub
Application.EnableEvents = False
If IsEmpty(Target.Value) Then
Target.Offset(, 1).Value = Empty
Else
Target.Offset(, 1).FormulaR1C1 = "=RC4*RC5"
End If
Application.EnableEvents = True
End Sub