Drum roll please ---
Thank you guys for comments and suggestions they did help me get to this point understanding what exactly was going on behind the scenes.
System.Data.DataTable dt = ds.Tables[0];
dt.Columns.Add("testrow", typeof(int));
DataRow dr = dt.NewRow();
int sum = 0;
for (int i = 1; i < dt.Columns.Count; i++)
{
for (int j = 1; j < dt.Rows.Count; j++)
{
if (j == dt.Rows.Count - 1)
{
dt.Rows[i][j] = Convert.ToInt32(sum);
sum = 0;
}
else
{
object number = dt.Rows[i][j];
sum += Convert.ToInt32(number);
}
}
dataGridView1.DataSource = dt;
}