9.2

Code below for the 9.2 task:

Option Strict On
Public Class Form1
    Dim Total As Integer
    Dim NumberOfMarks As Integer

    Private Sub btnOK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnOK.Click
        Dim Number As Integer
        Number = CInt(txtMark.Text)
        lstMarks.Items.Add(Number)
        Call ProcessOneNumber(Number, Total, NumberOfMarks)
        btnShowMean.Enabled = True
        txtMark.Text = ""
        txtMark.Focus()

    End Sub

    Private Sub btnShowMean_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnShowMean.Click
        Dim Mean As Double
        Call Calcmean(Total, NumberOfMarks, CSng(Mean))
        txtMean.Text = CStr(Mean)
        txtMean.Visible = True
        lblMean.Visible = True
        txtMark.Enabled = False
        btnOK.Enabled = False

    End Sub

    Private Sub btnQuit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnQuit.Click
        Me.Close()
    End Sub

    Private Sub txtMark_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtMark.TextChanged
        btnOK.Enabled = True

    End Sub
    Sub ProcessOneNumber(ByVal ExamMark As Integer, ByRef MarksTotal As Integer, ByRef CountOfMarks As Integer)
        MarksTotal = MarksTotal + ExamMark
        CountOfMarks = CountOfMarks + 1
    End Sub
    Sub Calcmean(ByVal MarksTotal As Integer, ByVal CountOfMarks As Integer, ByRef Average As Single)
        Average = CSng(MarksTotal / CountOfMarks)

    End Sub
End Class