table.Compute("AVG(Col3)", "Col1 = 'One' AND Col2 = 'Two'")

            DataTable table = new DataTable();
            table.Columns.Add("col1", typeof(string));
            table.Columns.Add("col2", typeof(string));
            table.Columns.Add("Value", typeof(decimal));
            table.Rows.Add("123", "1", 4.0M);
            table.Rows.Add("123", "1", 5.0M);
            table.Rows.Add("1", "123", 1.0M);
            table.Rows.Add("123", "2", 2.0M);
            table.Rows.Add("123", "2", 3.0M);

            var query = from row in table.AsEnumerable()
                        group row by new
                        {
                            col1 = row.Field<string>("col1"),
                            col2 = row.Field<string>("col2")
                        }
                        into grp
                        select new
                        {
                            Col1 = grp.Key.col1,
                            Col2 = grp.Key.col2,
                            Quantity = grp.Sum(r => r.Field<decimal>("Value")),
                            Average = grp.Average(r => r.Field<decimal>("Value"))
                        };

            Console.WriteLine("{0,5} {1,5} {2,5} {3,5}", "Col1", "Col2", "Quantity", "Average");
            foreach (var item in query)
            {
                Console.WriteLine("{0,5} {1,5} {2,6} {3,6}", item.Col1,
                item.Col2, item.Quantity, item.Average);
            }