using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var people = new[]

        {

            new Person("Ola", 175),

            new Person("Simon", 175),

            new Person("Bertil", 165),

            new Person("Ola", 175),

            new Person("Daniel", 175),

            new Person("Lennart", 144)

        };

        var mostCommonLength = people.GroupBy(x => x.Length).Select(x => new { Length = x.Key, Count = x.Count() }).OrderByDescending(x => x.Count).FirstOrDefault().Length;

        Console.WriteLine(mostCommonLength);

        var mostCommonNameInMostCommonLength = people.GroupBy(x => x.Length).Select(x => new { Length = x.Key, Name = x.GroupBy(y => y.Name).Select(y => new { Name = y.Key, Count = y.Count() }).OrderByDescending(y => y.Count).FirstOrDefault().Name }).OrderByDescending(x => x.Length).FirstOrDefault().Name;

        Console.WriteLine(mostCommonNameInMostCommonLength);

    }

}

​

public class Person

{

    public Person(string name, int length)

    {

        Name = name;

        Length = length;

    }

    public string Name { get; }

    public int Length { get; }

}