using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

​

public class Item

{

    public DateTime Date { get; set; }

    public double Amount { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        List<List<Item>> myListOfLists = new List<List<Item>>()

        {

            new List<Item>() 

            {

                new Item { Date=DateTime.Now.AddDays(-1), Amount=100 }, 

                new Item { Date=DateTime.Now.AddDays(-2), Amount=150 } 

            },

            new List<Item>() 

            {

                new Item { Date=DateTime.Now.AddDays(-1), Amount=100 }, 

                new Item { Date=DateTime.Now.AddDays(-3), Amount=400 } 

            },

            new List<Item>() 

            {

                new Item { Date=DateTime.Now.AddDays(-2), Amount=200 }, 

                new Item { Date=DateTime.Now.AddDays(-3), Amount=150 } 

            }

        };

        List<Item> result = myListOfLists.SelectMany(x => x)

                                     .GroupBy(x => x.Date)

                                     .Select(grp => new Item

                                     {

                                         Date = grp.Key,

                                         Amount = grp.Sum(y => y.Amount)

                                     }).ToList();

        foreach(var res in result)

            Console.WriteLine(res.Date.ToShortDateString()+" "+res.Amount);

    }

}