using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        //Console.WriteLine("Hello World");

        List<OneBillAccount> accounts = new List<OneBillAccount>();

        List<MobilityAccount>  mobAccounts = new List<MobilityAccount>();

​

        accounts.Add((new OneBillAccount()));

        mobAccounts.Add(new MobilityAccount{ AccountNumber = 555 });

        accounts.Add((new OneBillAccount{ MobilityAccounts = mobAccounts }));

        accounts.Find(x => x.MobilityAccounts.Any(y => y.AccountNumber == 555));

        Console.WriteLine(accounts.Count);

    }

    public class OneBillAccount 

    {

        public List<MobilityAccount> MobilityAccounts { get; set; }

    }

    public class MobilityAccount 

    {

                public int AccountNumber { get; set; }

    }

}