computeTip

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim sum As Double
        sum = CDbl(TextBox1.Text)
        If sum > 0 Then
            Dim tip As Double
            tip = sum * 0.1
            TextBox2.Text = CStr(tip)
        Else
            MessageBox.Show("Invalid Input")
        End If

    End Sub
End Class