var groupbyLinq = (from c in lst
                   group c by c.Name into g
                   from d in g
                   select d.Ave).ToList();

var groupbyRcesult = lst.GroupBy(c => c.Name);
List<int> lstAves = new List<int>();
foreach (var item in groupbyRcesult)
{
    lstAves.AddRange(item.Select(student => student.Ave));
}