Operations on XML data

Question

Hello,
I have a big problem.I created application in Visual studio on WPF, which can open and read XML files and show data in datagrid. But i cannot find out how to do mathematical operations with XML data.

My CODE
(XAML)
<Window x:Class="XMLReader.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="XMLReader" Height="370" Width="531">
    <Grid>

        <TextBox Margin="12,6,104,0" Name="FileNameTextBox" Height="32" VerticalAlignment="Top" />

        <Button Content="Open" Margin="0,6,10,0"

            Name="button1" Click="button1_Click" HorizontalAlignment="Right" Width="88" Height="32" VerticalAlignment="Top"/>

        <DataGrid AutoGenerateColumns="False" Margin="12,53,10,12" Name="dataGrid1" ItemsSource="{Binding Path=Elements[Car]}" Foreground="DarkBlue" HorizontalGridLinesBrush="#00000000" VerticalGridLinesBrush="#00000000" SelectionChanged="dataGrid1_SelectionChanged">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Model name &#xa;With VAT" Binding="{Binding Path=Attribute[Name].Value}" />
                <DataGridTextColumn Header="Without VAT" Binding="{Binding Path=Attribute[WithountDPH].Value}"/>
                </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>

(C#)
namespace XMLReader
{
    using System.Xml.Serialization;
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
       
        public void button1_Click(object sender, RoutedEventArgs e)
        {
            Microsoft.Win32.OpenFileDialog dlg = new Microsoft.Win32.OpenFileDialog();

            dlg.DefaultExt = ".xml";
            dlg.Filter = "Extended Mark Language (.xml)|*.xml";

            Nullable<bool> result = dlg.ShowDialog();

            if (result == true)
            {
                string filename = dlg.FileName;
                FileNameTextBox.Text = filename;
                var carlist = XElement.Load(filename);
                this.dataGrid1.DataContext = carlist;
            }         
        }
    }
}

Example of my XML
<Cars>
<Car Name = "FORD FIESTA" SaleDate = "16.12.2000" WithountDPH = "923550" DPH = " 20"/>
<Car Name = "FORD ESCORT/ESCORT DE LUXE/SUPER" SaleDate = "04.10.2014" WithountDPH = "938730" DPH = " 20"/>
<Car Name = "FORD C/CX TEN" SaleDate = "18.04.2010" WithountDPH = "755743" DPH = " 20"/>
<Car Name = "FORD CONSUL I/ZEPHYR I CONVERTIBLE" SaleDate = "05.01.2009" WithountDPH = "605291" DPH = " 20"/>
.
.
.
</Cars>

So my question is, how to multiply "WithoutDPH " column with "DPH" and show results in datagrid.
(DPH = VAT)