Public Class Form1
    Dim result, amount, con_rate As String


    Private Sub Label1_Click(sender As Object, e As EventArgs) Handles Label1.Click

    End Sub

    Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged

    End Sub

    Private Sub TextBox2_TextChanged(sender As Object, e As EventArgs) Handles TextBox2.TextChanged

    End Sub

    Private Sub TextBox3_TextChanged(sender As Object, e As EventArgs) Handles TextBox3.TextChanged

    End Sub

    Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
        If ComboBox1.SelectedIndex = 0 Then
            con_rate = 0.9
            TextBox2.Text = con_rate
        End If

        If ComboBox1.SelectedIndex = 1 Then
            con_rate = 1.11
            TextBox2.Text = con_rate
        End If

        If ComboBox1.SelectedIndex = 2 Then
            con_rate = 0.79
            TextBox2.Text = con_rate
        End If

        If ComboBox1.SelectedIndex = 3 Then
            con_rate = 1.26
            TextBox2.Text = con_rate
        End If

    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        amount = Val(TextBox1.Text)
        TextBox3.Text = (Val(TextBox1.Text) * Val(TextBox2.Text))
        If Not amount = IsNumeric(amount) Then
            MsgBox("Put correct input")
        End If

    End Sub

End Class