TaxCalculator.cs

public class Tests
{
	[Fact]
	public void Tax10()
	{
		var calc = new TaxCalculator();

		decimal tax = calc.Tax(5000);
		Assert.Equal(500, tax);
	}

	[Fact]
	public void Tax14()
	{
		var calc = new TaxCalculator();

		decimal tax = calc.Tax(5800);

		Assert.Equal(609.2m, tax);
	}

	[Fact]
	public void Tax23()
	{
		var calc = new TaxCalculator();

		decimal tax = calc.Tax(9000);

		Assert.Equal(1087.8m, tax);
	}

	[Fact]
	public void Tax30()
	{
		var calc = new TaxCalculator();

		decimal tax = calc.Tax(15000);

		Assert.Equal(2532.9m, tax);
	}

	[Fact]
	public void Tax45()
	{
		var calc = new TaxCalculator();

		decimal tax = calc.Tax(50000);

		Assert.Equal(15068.1m, tax);
	}
}

public class TaxCalculator
{
	private readonly IEnumerable<Tuple<decimal, decimal>> taxRates = new[]
		                                                                {
		                                                                 	Tuple.Create(40230m, .45m),
		                                                                 	Tuple.Create(21240m, .33m),
		                                                                 	Tuple.Create(14070m, .30m),
		                                                                 	Tuple.Create(8660m, .23m),
		                                                                 	Tuple.Create(5070m, .14m),
		                                                                 	Tuple.Create(0m, .1m),
		                                                                };

	public decimal Tax(decimal sum)
	{
		decimal result = 0m;
		decimal x = sum;
		foreach (var taxRate in taxRates)
		{
			if (x > taxRate.Item1)
			{
				result += (x - taxRate.Item1)*taxRate.Item2;
				x = taxRate.Item1;
			}
		}
		return result;
	}
}