VB Code for Average Test mark

Public Class Form1
    Dim Mark As Decimal
    Dim Listlength As Integer

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
        Mark = TextBox1.Text
        ListBox1.Items.Add(Mark)
        Listlength = Listlength + 1
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
        Dim OneMark As Integer
        Dim Total As Decimal
        Dim count As Integer
        Dim Average As Decimal
        Do
            ListBox1.SelectedIndex = count
            OneMark = ListBox1.SelectedItem
            Total = Total + OneMark
            count = count + 1
        Loop Until count = Listlength

        Average = Total / count
        TextBox2.Text = Average

    End Sub
End Class