//15

string wod = "00:00-01:00,00:30-02:30,01:30-03:00,02:00-04:00,02:30-05:00";

string[] schel = wod.Split(',');
int[] time1 = new int[24];

foreach (string schedule in schel)
{
    string[] times = schedule.Split('-');
    string[] startTime = times[0].Split(':');
    string[] endTime = times[1].Split(':');

    int hour1 = int.Parse(startTime[0]);
    int hour2 = int.Parse(endTime[0]);

    for (int i = hour1; i < hour2; i++)
    {
        time1[i]++;
    }
}

int maxTime = 0;
for (int i = 0; i < 24; i++)
{
    if (time1[i] > maxTime)
    {
        maxTime = time1[i];
    }
}

Console.WriteLine("Максимальное количество касс работает в интервале: ");
for (int i = 0; i < 24; i++)
{
    if (time1[i] == maxTime)
    {
        Console.WriteLine("{0:00}:00 - {1:00}:00", i, i + 1);
    }
}