Solution to Jane Street "Think of the Children" Puzzle : https://www.janestreet.com/puzzles/think-of-the-children/

ThinkOfTheChildren.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ThinkOfTheChildren
{
    public class Program
    { 
    public static void Main(string[] args)
    {
            var x = Enumerable.Range(1, 18)
                .Select(i => new
                {
                    i = i,
                    jRange = Enumerable.Range(1, i)
                })
                .SelectMany(e => e.jRange, (e, j) => new
                {
                    i = e.i,
                    j = j,
                    kRange = Enumerable.Range(1, j)
                })
                .SelectMany(e => e.kRange, (e, k) => new int[]
                { e.i, e.j, k })
                .GroupBy(triple => triple, new Comparer())
                .Where(group => group.Count() > 1 &&
                                group
                                    .Where(triple => triple[0] > triple[1])
                                    .Count() == 1)
                .First(group => group.Key
                    .Product() > 72);

            Console.WriteLine(x.Key.Product());

    }
}

    public class Comparer : IEqualityComparer<int[]>
    {
        public bool Equals(int[] a1, int[] a2)
        {
            return a1.Product() == a2.Product() &&
                   a1.Sum() == a2.Sum();
        }

        public int GetHashCode(int[] a)
        {
            return 1;
        }
    }

    public static class Extensions
    {
        public static int Product(this int [] arr)
        {
            return arr.Aggregate((product, next) => product * next);
        }
    }
}