using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        for (decimal i = 0; i < 100; i++) {

            var amount = i + .8M;

            Console.Write(amount.ToString("C2") + " => ");

            var cashAmounts = GetCashAmounts(amount).OrderBy(a => a).Distinct().Take(3).ToList();

            cashAmounts.ForEach(a => Console.Write(a.ToString("C2") + " "));

            Console.WriteLine("");

        }

    }

    private static IEnumerable<decimal> GetCashAmounts(decimal amount) {

        List<decimal> billsYo = new List<decimal>() { 1, 5, 10, 20, 50, 100 };

        var roundedUp = Math.Ceiling(amount);

        var lastBill = 0M;

        foreach (var bill in billsYo) {

            yield return GetFirstMultipleThatCovers(bill, roundedUp);

            if (lastBill == 20)

            {

                if (roundedUp > bill && lastBill + bill >= roundedUp)

                    yield return lastBill + bill;

                else if (roundedUp > bill && lastBill * 2 + bill >= roundedUp)

                    yield return lastBill * 2 + bill;

            }

            lastBill = bill;

        }

    }

    private static decimal GetFirstMultipleThatCovers(decimal bill, decimal givenAmount) {

        int i = 1;

        while (i * bill < givenAmount) {

            i++;

        }

        return i* bill;

    }   

}