cal. qty * rate=amount datagridtextcolum in datagrid EF

Question

<DataGrid AutoGenerateColumns="False" 
                  Grid.Row="2"  
                  HorizontalAlignment="Stretch" 
                  Margin="0,2,0,0" 
                  Name="CenterGrid"
                  IsSynchronizedWithCurrentItem="True"
                  VerticalAlignment="Top"
                  CanUserAddRows="True"
                  CanUserDeleteRows="True" 

                  ItemsSource="{Binding Source={StaticResource DetailsViewSource}}"  >

            <DataGrid.Columns>

                <DataGridComboBoxColumn x:Name="cmbCentername" Header="Item Name"
                                           ItemsSource="{Binding Source={StaticResource ItemList}}"
                                            SelectedItemBinding="{Binding Path=ItemMaster}"
                                            DisplayMemberPath="ItemName"
                                            SelectedValuePath="ItemId">
                    <DataGridComboBoxColumn.EditingElementStyle>
                        <Style TargetType="ComboBox">
                            <Setter Property="IsEditable" Value="True" />
                            <Setter Property="IsDropDownOpen" Value="True"/>
                        </Style>
                    </DataGridComboBoxColumn.EditingElementStyle>
                </DataGridComboBoxColumn>


                <DataGridTextColumn  x:Name="txtqty" Header="Qty" Width="*" Binding="{Binding Path=Qty}"/>
                <DataGridTextColumn  x:Name="txtrate" Header="Rate" Width="*" Binding="{Binding Path=Rate}"/>

                <DataGridTextColumn  x:Name="txtamount" Header="Amount" Width="*" SortMemberPath="Amount"  >

                    <DataGridTextColumn.Binding>
                        <MultiBinding Mode="OneWay"  Converter="{StaticResource AmountConverters}">
                                <Binding Path="Qty"/>
                            <Binding Path="Rate"/>
                            </MultiBinding>
                    </DataGridTextColumn.Binding>

                </DataGridTextColumn>

            </DataGrid.Columns>
        </DataGrid> 

Detailviewsource is Collectionviewsource ..

i am doing this project using Entity Framework and wpf

i want want to multiply qty* rate = amount runtime with binding and i want to bind Amount Property of Detailviewsource because it can be send automatically send in to database.. how to do it Using Multibinding and Without Multibinding Or any other Procedure..

in this image amount is manully type