public interface ITax
{
    decimal ProvincialTaxRate { get; set; } // Yes, I'm Canadian :)
    decimal CalculateTax(decimal subtotal);
}

public SaskatchewanTax
{
    public decimal ProvincialTaxRate { get; set; }

    public SaskatchewanTax()
    {
        ProvincialTaxRate = new decimal(0.05f);
    }

    public decimal CalculateTax(subtotal)
    {
        return ProvincialTaxRate * subtotal + FederalTaxRate * subtotal;
    }
}

public OntarioTax
{
    public decimal ProvincialTaxRate { get; set; }

    public OntarioTax()
    {
        ProvincialTaxRate = new decimal(0.08f);
    }

    public decimal CalculateTax(decimal subtotal)
    {
        return ProvincialTaxRate * subtotal + FederalTaxRate * subtotal;
    }
}

public class TaxCalculator
{
    public static decimal FederalTaxRate = new decimal(0.05f);
}

public interface ITax
{
    decimal CalculateTax(decimal subtotal);
}

public class SaskatchewanTax : ITax
{
    private readonly decimal provincialTaxRate;
    private readonly decimal federalTaxRate;

    public SaskatchewanTax(decimal federalTaxRate)
    {
        provincialTaxRate = 0.05m;
        this.federalTaxRate = federalTaxRate;
    }

    public decimal CalculateTax(decimal subtotal)
    {
        return provincialTaxRate * subtotal + federalTaxRate * subtotal;
    }
}

public class OntarioTax : ITax
{
    private readonly decimal provincialTaxRate;
    private readonly decimal federalTaxRate;

    public OntarioTax(decimal federalTaxRate)
    {
        provincialTaxRate = 0.08m;
        this.federalTaxRate = federalTaxRate;
    }

    public decimal CalculateTax(decimal subtotal)
    {
        return provincialTaxRate * subtotal + federalTaxRate * subtotal;
    }
}

public abstract class Tax
{
    protected decimal ProvincialTaxRate; // Yes, you are Canadian ;)
    public decimal CalculateTax(decimal subtotal)
    {
        return ProvincialTaxRate * subtotal + FederalTaxRate * subtotal;
    }
    decimal FederalTaxRate = new decimal(0.20f);
}

public class SaskatchewanTax : Tax
{
    public SaskatchewanTax()
    {
        base.ProvincialTaxRate = new decimal(0.05f);
    }

}

public class OntarioTax : Tax
{
    public OntarioTax()
    {
        base.ProvincialTaxRate = new decimal(0.08f);
    }

}

public decimal CalculateTax(decimal subtotal, decimal provincialTaxRate, decimal federalTaxRate) {
    return provincialTaxRate * subtotal + federalTaxRate * subtotal;
    }