ayendeQuiz

void Main()
{
    var examples = new List<double> { 5000, 5800, 9000, 15000, 50000};
    var results = new Dictionary<double, double>();
    var dict = BuildTaxBrackets();
    var taxCalculator = new TaxCalculator();

    examples.ForEach(amount => results.Add(amount, taxCalculator.CalculateTaxAmount(amount, dict)));
    results.Dump("Results");
}

internal class TaxBracket
{
    public double MinAmount { get; set; }
    public double MaxAmount { get; set; }
    public double TaxRate { get; set; }
}

// Define other methods and classes here
internal class TaxCalculator
{
    public double CalculateTaxAmount(double amount, Dictionary<int, TaxBracket> dict)
    {
        var ratesToApply = dict
            .Where(x => x.Value.MinAmount <= amount)
            .OrderByDescending(x => x.Key)
            .ToDictionary(x => x.Key, x => x.Value);

        var runningTotalTaxAmount = 0d;

        if (!ratesToApply.Any())
        {
            return 0d;
        }

        if (ratesToApply.Count() == 1)
        {
            return ratesToApply[0].TaxRate * amount;
        }

        for (var i = ratesToApply.Count() - 1; i > -1; i--)
        {
            var rate = ratesToApply[i];
            var minAmount = i == 0 ? ratesToApply[i].MinAmount : ratesToApply[i-1].MaxAmount;
            var amountToTaxInBracket = amount - minAmount;

            runningTotalTaxAmount += rate.TaxRate * amountToTaxInBracket;
            amount -= amountToTaxInBracket;
        }

        return runningTotalTaxAmount;
    }
}

internal Dictionary<int, TaxBracket> BuildTaxBrackets()
{
    return new Dictionary<int, TaxBracket>
    {
        {0,
        new TaxBracket
        {
            MinAmount = 0,
            MaxAmount = 5070,
            TaxRate = .10
        }},
        {1,
        new TaxBracket
        {
            MinAmount = 5071,
            MaxAmount = 8660,
            TaxRate = .14
        }},
        {2,
        new TaxBracket
        {
            MinAmount = 8661,
            MaxAmount = 14070,
            TaxRate = .23
        }},
        {3,
        new TaxBracket
        {
            MinAmount = 14071,
            MaxAmount = 21240,
            TaxRate = .30
        }},
        {4,
        new TaxBracket
        {
            MinAmount = 21241,
            MaxAmount = 40230,
            TaxRate = .33
        }},
        {5,
        new TaxBracket
        {
            MinAmount = 40231,
            MaxAmount = double.PositiveInfinity,
            TaxRate = .45
        }},
    };
}