If you're drawing the data for the first two columns out of a database, just add a third column into the SQL query that combines the two. Then databind the gridview to the resulting table.
Select exam1, exam2, SUM(exam1 + exam2) as total from table;
If you need to do it after the query for some reason, add a new column to the resulting dataTable from the query and then loop through inserting the sum of the two columns together. Then databind the gridview once that is done.