void Main()
{
    DateTime start = new DateTime(2020, 4, 3);
    DateTime end = new DateTime(2020, 4, 30);

    List<(DateTime, DateTime)> pauses = new();
    pauses.Add((new DateTime(2020, 4, 6), new DateTime(2020, 4, 12)));

    Calculate(start, end, pauses).Dump();
}

int CalculaQte(DateTime start, DateTime end, IEnumerable<(DateTime, DateTime)> pauses)
{
    const int numberBefore20 = 200;
    const int numberAfter20 = 300;
    int sum = 0;

    for (DateTime dt = start; dt < end; dt = dt.AddDays(1))
    {
        if (dt.DayOfWeek == DayOfWeek.Sunday || dt.DayOfWeek == DayOfWeek.Saturday)
        {
            continue;
        }

        if (pauses.Any(p => dt > p.Item1 && dt < p.Item2))
        {
            continue;
        }

        sum += dt < new DateTime(2020, 4, 20) ? numberBefore20 : numberAfter20;
    }

    return sum;
}