How to replace something in a listbox using substrings?

Hi, I have made a code where I write a recipe to a file and then I can read from the file and it is output in a listbox. I need to allow the user to change the amount of people served and then the quantities change to suit the serving number. The serving number is input and the rest of the code works apart from the changing the quantities bit.

The code that I am using is below. I have no idea if i'm doing it right though.

Private Sub btnOKServed_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnOKServed.Click
                lstExistingRecipe.Items.Clear()
                Dim FileReader As New IO.StreamReader("E:\" & lstRecipeChoose.Text & ".txt")
                Dim charcounter As Double
                Do While FileReader.Peek <> -1
                    StrEntry = FileReader.ReadLine().ToString
                    If StrEntry.StartsWith(Ingredients & "Quantity and units: ") Then
                        Quantity = StrEntry.Substring(charcounter + 20, 3)
                        NewIngredientsAndUnits = StrEntry.Substring(charcounter + 24)
                        Quantity = lstExistingRecipe.Text / PeopleServed * NewPeopleServed
                        NewRecipe = Ingredients & "Quantity and units: " & Quantity & NewIngredientsAndUnits
                        lstExistingRecipe.Items.Add(NewRecipe)
                    End If

                Loop
            End Sub

Thanks in advance!

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