Public Class Form1

    Private Sub GetFactorial(ByRef n As Integer)
        Dim product As Integer = 1

        Do While n > 1
            product = product * n
            n = n - 1
        Loop

        n = product
    End Sub

    Private Sub btnFact_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnFact.Click
        Dim num As Integer = Val(tbNum.Text)

        GetFactorial(num)

        MessageBox.Show(num)
    End Sub

    Private Sub btnCombos_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCombos.Click
        ' The number of combinations is calculated using 
        ' the following formula:   N! / ( R! * (N-R)! )
        Dim n As Integer = Val(tbN.Text)
        Dim r As Integer = Val(tbR.Text)
        Dim nMinusR As Integer = n - r

        GetFactorial(n)
        GetFactorial(r)
        GetFactorial(nMinusR)

        Dim numCombinations As Integer
        numCombinations = n / (r * nMinusR)

        MessageBox.Show(numCombinations)
    End Sub
End Class