7.3.1

    class GroupResults
    {
        public string numberOfGroup;
        public double[,] x;
    }
    class AverageResults : GroupResults
    {
        public double sred;
        public AverageResults(string numberOfGroup, double[,] x)
        {
            sred = 0;
            int cnt = 0;
            this.numberOfGroup = numberOfGroup;
            this.x = x;
            for (int i = 0; i < x.GetLength(0); i++)
            {
                double allMarks = 0;
                int k = 0;
                for (int j = 0; j < x.GetLength(1); j++)
                {
                    allMarks += x[i, j];
                    if (x[i, j] == 2)
                    {
                        k++;
                    }
                }
                if (k == 0)
                {
                    sred += allMarks;
                    cnt += 5;
                }
            }
            sred /= cnt;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            AverageResults[] gr = new AverageResults[3];
            gr[0] = new AverageResults("Первая группа", new double[,] { { 3, 5 ,5, 4, 5} ,
                                                                      { 2, 4, 4, 3, 5} ,
                                                                      { 5, 5, 5, 4, 5} ,
                                                                      { 4, 5, 3, 3, 5} });
            gr[1] = new AverageResults("Вторая группа", new double[,] { { 5, 5 ,5, 3, 5} ,
                                                                      { 3, 5, 4, 3, 5} ,
                                                                      { 5, 3, 5, 4, 5} ,
                                                                      { 4, 3, 3, 4, 5} });
            gr[2] = new AverageResults("Третья группа", new double[,] { { 4, 2 ,5, 4, 5} ,
                                                                      { 3, 4, 4, 5, 5} ,
                                                                      { 5, 5, 5, 4, 5} ,
                                                                      { 4, 5, 5, 5, 5} });
            for (int i = 0; i < gr.Length - 1; i++)
            {
                double amax = gr[i].sred;
                int imax = i;
                for (int j = i + 1; j < gr.Length; j++)
                {
                    if (gr[j].sred > amax)
                    {
                        amax = gr[j].sred;
                        imax = j;
                    }
                }
                AverageResults temp;
                temp = gr[imax];
                gr[imax] = gr[i];
                gr[i] = temp;
            }
            for (int i = 0; i < gr.Length; i++)
                WriteLine(gr[i].sred);
        }
    }