And exercise for a user in a C# Telegram group.

ClosestSolution.cs

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

namespace ConsoleApp1
{
    internal interface Solution
    {
        /// <summary>
        /// Simple interface for solutions to math problems with 2 factors
        /// </summary>
        int Factor1 { get; set; }
        int Factor2 { get; set; }
        int Target { get; set; }
        double DistanceToTarget { get; }
        string Visualization { get; }
    }

    internal class Multiplication : Solution
    {
        /// <summary>
        /// Multiplies factor1 and factor 2
        /// </summary>
        public int Factor1 { get; set; }
        public int Factor2 { get; set; }
        public int Target { get; set; }
        public double DistanceToTarget => System.Math.Abs(Target - (Factor1 * Factor2));
        public string Visualization => $"{Factor1} * {Factor2} = {Factor1 * Factor2}";
    }

    internal class Division : Solution
    {
        /// <summary>
        /// Divides factor1 over factor 2
        /// </summary>
        public int Factor1 { get; set; }
        public int Factor2 { get; set; }
        public int Target { get; set; }
        public double DistanceToTarget => System.Math.Abs(Target - (Factor1 / Factor2));
        public string Visualization => $"{Factor1} / {Factor2} = {Factor1 / Factor2}";
    }

    internal class Addition : Solution
    {
        /// <summary>
        /// Adds factor1 to factor 2
        /// </summary>
        public int Factor1 { get; set; }
        public int Factor2 { get; set; }
        public int Target { get; set; }
        public double DistanceToTarget => System.Math.Abs(Target - (Factor1 + Factor2));
        public string Visualization => $"{Factor1} + {Factor2} = {Factor1 + Factor2}";
    }

    internal class Subtraction : Solution
    {
        /// <summary>
        /// Subtracts factor2 from factor1
        /// </summary>
        public int Factor1 { get; set; }
        public int Factor2 { get; set; }
        public int Target { get; set; }
        public double DistanceToTarget => System.Math.Abs(Target - (Factor1 - Factor2));
        public string Visualization => $"{Factor1} - {Factor2} = {Factor1 - Factor2}";
    }

    class Program
    {
        /// <summary>
        /// Simple program that tries to find solutions that come as close as posible to a target number. 
        /// </summary>
        /// <param name="args"></param>
        static void Main(string[] args)
        {
            // The factors we'll use
            var factors = new[] { 1, 3, 3, 7, 10, 5 };
            // The number we're trying to be close to 
            const int target = 999;
            // Our solutions
            var solutions = new List<Solution>();
            // The factors we've already used so we won't end up with duplicate solutions
            var usedFactors = new List<int>();

            foreach (var factor1 in factors)
            {
                // Avoiding duplicate solutions
                foreach (var factor2 in factors.Where(x => !usedFactors.Contains(x)))
                {
                    // Add each type of solution
                    solutions.AddRange(new Solution[]
                    {
                        // A constructor would be nice, but whatever. 
                        new Multiplication { Factor1 = factor1, Factor2 = factor2, Target = target },
                        new Division { Factor1 = factor1, Factor2 = factor2, Target = target },
                        new Addition { Factor1 = factor1, Factor2 = factor2, Target = target },
                        new Subtraction { Factor1 = factor1, Factor2 = factor2, Target = target }
                    });
                }
                
                usedFactors.Add(factor1);
            }

            foreach (var solution in solutions.OrderBy(x => x.DistanceToTarget).Take(10))
            {
                Console.WriteLine(solution.Visualization);
            }
            Console.WriteLine("Press any key to exit");
            Console.ReadKey();
        }
    }
}