Finding similar jugs in O(n2) time. This is a naive implementation of finding two jugs of similar capacity.

SimilarJugs.cs

namespace dotNetPlayGround
{
    using System;
    using System.Collections.Generic;
    
    public struct Jug
        {
            public int Capacity;
            public int Filled;
            public Jug(int capacity,int filled){
                Capacity = capacity;
                Filled = filled;
            }
            public override bool  Equals(object obj)
            {
                Jug jug = (Jug)obj;
                return (this.Filled + jug.Filled == this.Capacity && this.Filled + jug.Filled == jug.Capacity);
            }
        }
        public class WaterJugs
        {
            static void Main()
            {
                int total = 4;
                Jug[] red = { new Jug(10, 5),new Jug(20,10),new Jug(30,15), new Jug(40,20) };
                Jug[] blue = { new Jug(40, 20), new Jug(30, 15), new Jug(20, 10), new Jug(10, 5) };
    
                for (int i = 0; i < total; i++)
                {
                    for (int j = 0; j < total; j++)
                    {
                        if (red[i].Equals(blue[j]))
                            Console.WriteLine("group:({0},{1})", i, j);
                    }
                }
            }
        }
}