Hi,
How about the following?
(from d in dt.AsEnumerable()
group d by Tuple.Create(d["Nombre del elemento nominado"],d["ID de producto de crudo"]) into grp
select dtResult.Rows.Add(new object[] { grp.Key.Item1,grp.Key.Item2,grp.Sum(r=>Int32.Parse(r["Barriles promedio por día nominados"].ToString())/total)})
).CopyToDataTable()
Regards,