using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var source = new List<string>

        {

            "H,1000,28-09-2017", 

            "L,30,Pete", 

            "L,50,Pete", 

            "H,200,29-09-2017", 

            "L,35,Pete", 

            "L,45,Pete", 

            "L,55,Pete", 

        };

        IEnumerable<string> linesWithCountsAndSums = source

            .Select((value, index) => new { value, index })

            .Where(pair => pair.value[0] == 'H')

            .Select(pair => new { pair.value, lines = source.Skip(pair.index + 1).TakeWhile(v => v[0] != 'H') })

            .Select(pair => new { header = $"{pair.value},{pair.lines.Count()},{pair.lines.Sum(ExtractLineValue)}", lines = pair.lines })

            .SelectMany(pair => Enumerable.Repeat(pair.header, 1).Concat(pair.lines));

        foreach (string line in linesWithCountsAndSums)

        {

            Console.WriteLine(line);

        }

    }

​

    static int ExtractLineValue(string line)

    {

        int index = line.IndexOf(',', 2);

        if (index == -1) index = line.Length;

        string part = line.Substring(2, index - 2);

        int.TryParse(part, out int result);

        return result;

    }

}