Ayende tax calculation challenge

I had some fun doing this little challenge from ayende, it's a good exercise to practice TDD. Here is my solution in C#.
The tests

[TestFixture]
public class TaxCalculatorTests
{
	[Test]
	public void Tax_for_0()
	{
		var calculator = new TaxCalculator();
		
		var tax = calculator.TaxFor(0);
		
		Assert.That(tax, Is.EqualTo(0));
	}
			
	[Test]
	public void Tax_for_5000()
	{
		var calculator = new TaxCalculator();
		
		var tax = calculator.TaxFor(5000);
		
		Assert.That(tax, Is.EqualTo(500));
	}
	
	[Test]
	public void Tax_for_5800()
	{
		var calculator = new TaxCalculator();
		
		var tax = calculator.TaxFor(5800);
		
		Assert.That(tax, Is.EqualTo(609.2).Within(0.001));
	}

	[Test]
	public void Tax_for_9000()
	{
		var calculator = new TaxCalculator();
		
		var tax = calculator.TaxFor(9000);
		
		Assert.That(tax, Is.EqualTo(1087.8).Within(0.001));
	}

	[Test]
	public void Tax_for_15000()
	{
		var calculator = new TaxCalculator();
		
		var tax = calculator.TaxFor(15000);
		
		Assert.That(tax, Is.EqualTo(2532.9).Within(0.001));
	}

	[Test]
	public void Tax_for_50000()
	{
		var calculator = new TaxCalculator();
		
		var tax = calculator.TaxFor(50000);
		
		Assert.That(tax, Is.EqualTo(15068.1).Within(0.001));
	}
}

And the implementation

public class TaxCalculator
{
	private readonly Tuple<decimal, decimal>[] slices = 
	{
		new Tuple<decimal, decimal>(40230, 0.45M),
		new Tuple<decimal, decimal>(21240, 0.33M),
		new Tuple<decimal, decimal>(14070, 0.30M),
		new Tuple<decimal, decimal>(8660, 0.23M),
		new Tuple<decimal, decimal>(5070, 0.14M),
		new Tuple<decimal, decimal>(0, 0.10M),
	};
	
	public decimal TaxFor(decimal sum)
	{
		if (sum == 0)
		{
			return 0;
		}
		
		var tax = 0M;
		foreach (var slice in slices)
                {
			if (sum > slice.Item1)
                        {
				var amount = sum - slice.Item1;
				tax += amount * slice.Item2;
				sum -= amount;
			}
		}
		
		return tax;
	}
}