Distance.cs

public class Distance
{
    public static decimal MilesToKilometers(decimal miles)
    {
        return miles * 1.60934m;
    }
}

public class TestDeterministic
{
    [Fact]
    public void Deterministic()
    {
        var result = Distance.MilesToKilometers(15);
        result.ShouldBe(24.14010m);
    }
}