using System.Globalization;
var notasAlunos = new Dictionary<string, List>();
notasAlunos.Add("Maria", new List { 5.5, 2.8, 7.5, 6 });
var notasAlunos2 = new Dictionary<string, List>
{
{ "João", new List {8, 9.5, 5.2, 6} }
};
double mediaAluno1 = notasAlunos["Maria"].Average(), mediaAluno2 = notasAlunos2["João"].Average();
Console.WriteLine(mediaAluno1.ToString("F2", CultureInfo.InvariantCulture));
Console.WriteLine(mediaAluno2.ToString("F2", CultureInfo.InvariantCulture));
