Public Class Form1


    Private Sub btncalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handlesbtncalculate.Click

        Dim bytepercentage As Byte

        lbloutput.Visible = True
        lbloutput.Text = txtinput.Text / txtfullmark.Text * 100
        bytepercentage = lbloutput.Text
        Select Case bytepercentage
        End Select

        If txtinput.Text > txtfullmark.Text Then
            lbloutput.Text = "Error your input is invalid"
            lbloutput.Visible = False

        ElseIf lbloutput.Text >= 80 Then
            lblgrade.Text = ("A")
        ElseIf lbloutput.Text >= 70 Then
            lblgrade.Text = ("B")
        ElseIf lbloutput.Text >= 60 Then
            lblgrade.Text = ("C")
        ElseIf lbloutput.Text >= 50 Then
            lblgrade.Text = ("D")
        ElseIf lbloutput.Text <= 50 Then
            lblgrade.Text = ("E")
        End If
    End Sub
 end class