Hello Hossam,

This could be achieved by handling the CellValueChanged event on the client side, the cell could be checked if it is from the Amount column and after that the needed changes could be done with the following code:

function CellValueChanged(sender, eventArgs)
            {
                var currentCell = eventArgs.get_cell(); 
                var rowIndex = currentCell.get_row().get_index();                
                var grid = $find("WebHierarchicalDataGrid1");

                if (currentCell.get_column().get_key() == "UnitPrice") {
                    // I assume the indexes are matching the one in the screenshot provided in the post 
                    //calculate total based on amount and rate
                    var total = currentCell.get_value() * grid.get_gridView().get_rows().get_row(rowIndex).get_cell(4).get_value();
                    //set the valuue of the total column
                     grid.get_gridView().get_rows().get_row(rowIndex).get_cell(5).set_value(total);
                    // calculate the sub total and populate the sub total field
                    //if you need to iterate the rows collection you can get its lenght with the following line:
                    //grid.get_gridView().get_rows().get_length()
                    document.getElementById("SubTotalTextBox1").value = total;
                }
            }

If you need some more information regarding the Client-Side Object Model you can review all the properties, methods and events of the WebHierarchicalDataGrid here:

https://www.infragistics.com/help/aspnet/webhierarchicaldatagrid~infragistics.web.ui.webhierarchicaldatagrid_members

Please let me know if you have any questions.

Regards,
Ivan Kitanov