Expressing A Repeating Decimal Number As A Fraction

[daydee]

Hi there!
Recently, someone asked me to come up with a small app that could simply add 2 fractions (with sums being inferior to 1) and display the result as a fraction as well (not in decimal form).
I thought this was going to be easy at first but I got stumped early on with this. Actually adding the 2 fractions is nothing but getting the result as a fraction is another matter.
For the most part I got it working using the function below which works well on decimal numbers without long repeating patterns, but on a fractions such as 1/7 with a decimal equivalent of 0.142857142857142857142857142857143 the function gracefully fails.
I guess what I need is a formula (preferably in VB) that would reduce those long repeating patterns within the numerator and getting the denominator value right before calling the function.

Again, as this sample all I'm trying to achieve is to come up with and display the 17/21 in my MessageBox:
1 2 17
-+-=-
7 3 21
Math was never my forte in school and I wish now that I would have paid a little more attention during my math class
Anyway thanks in advance!

Here's what I got so far:

Private Sub Command1_Click()
Dim lngNumerator As Long, lngDenominator As Long, lngCommonFactor As Long
 
lngNumerator = 5 
lngDenominator = 10
 
If GetCommonFactor(lngNumerator, lngDenominator, lngCommonFactor) Then
     MsgBox lngNumerator & " / " & lngDenominator
End If
 
End Sub
 
Public Function GetCommonFactor(Numerator As Long, Denominator As Long, CommonFactor As Long) As Boolean
    Dim lngIdx As Long
    
    CommonFactor = 1
    
    For lngIdx = Numerator To 2 Step -1
        If Numerator Mod lngIdx = 0 Then
            If Denominator Mod lngIdx = 0 Then
                CommonFactor = lngIdx
                Numerator = Numerator / CommonFactor
                Denominator = Denominator / CommonFactor
                GetCommonFactor = True
                Exit For
            End If
        End If
    Next
 
End Function