title: Date table column Sum and differencer
body: <p>i need to make a data table like that: </p>
  
  <pre><code>Subjects          old        new    diff
  Sub_1             10         50     40
  Sub_2             30         10     -20
  total             40         60     20
  </code></pre>
  
  <p>this is a part of code </p>
  
  <pre><code>DataTable subjects = new DataTable();
  subjects.Columns.Add("Subjects");
  subjects.Columns.Add("old");
  subjects.Columns.Add("new");
  subjects.Columns.Add("diff");
  subjects.Rows.Add("Sub_1", sub1.Old, sub1.New, (sub1.New - sub1.Old));
  subjects.Rows.Add("Sub_2", sub2.Old, sub2.New, (sub2.New - sub2.Old));
  subjects.Rows.Add("Total", .. total of above    .. , .. total of above    .., .. total of above  ..);
  </code></pre>
  
  <p>so i need to  ask how to calculate the total value of last column ( Total) , and is there is any other way to calculate the 4th coulmn ( 3rdcol - 2 2nd col ) </p>