LINQ group by question

group by person.cn.Split(separators)[0]

group by person.cn.Substring(0,5)

string oneSpace = " ";
string fiftySpace = "                                                  ";

var query =
  from person in db.Persons
  let lastname = person.cn.Replace(oneSpace, fiftySpace).SubString(0, 50).Trim()
  group person by lastname into g
  select new { Key = g.Key, Count = g.Count };