A new policy implements tax computation differently and not straightforwardly. While the common tax computation uses a solid percentage (e.g., 10%, 11%, 15%, etc.), Indonesia recently released a new tax computation using mixed percentages (e.g., 11%/12%). 

Here is the illustration to give you: 

• Selling (Untaxed) amount= $100
• Base Tax amount= Untaxed amount * (11%/12%) = $100 * (11%/12%) = $91.67

How can the tax computation be implemented to compute the base tax amount following the above use case?