using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var products = new List<Product>()

        {new Product() {ProductID = 1, ShopID = 1, Amount = 1000, Price = 220, AddedDate = DateTime.Parse("2020-07-01")}, 

         new Product() {ProductID = 1, ShopID = 1, Amount = 1000, Price = 230, AddedDate = DateTime.Parse("2020-07-30")}, 

         new Product() {ProductID = 1, ShopID = 1, Amount = 100, Price = 21, AddedDate = DateTime.Parse("2020-07-10")},

         new Product() {ProductID = 1, ShopID = 1, Amount = 100, Price = 22, AddedDate = DateTime.Parse("2020-07-31")}, 

​

         new Product() {ProductID = 1, ShopID = 2, Amount = 100, Price = 23, AddedDate = DateTime.Parse("2020-07-10")},

         new Product() {ProductID = 1, ShopID = 2, Amount = 100, Price = 20, AddedDate = DateTime.Parse("2020-07-12")},

        };

        var result = products.Where(x => x.ProductID == 1).OrderByDescending(x => x.AddedDate).ThenBy(x => x.Price / x.Amount).FirstOrDefault();

        Console.WriteLine(result.ProductID);

        Console.WriteLine(result.ShopID);

        Console.WriteLine(result.Amount);

        Console.WriteLine(result.Price);

        Console.WriteLine(result.AddedDate);

    }

}

​

internal class Product

{

    public int ProductID

    {

        get;

        set;

    }

​

    public int ShopID

    {

        get;

        set;

    }

​

    public decimal Amount

    {

        get;

        set;

    }

​

    public decimal Price

    {

        get;

        set;

    }

​

    public DateTime AddedDate

    {

        get;

        set;

    }

}