Calculate Two Different Prices Based On A Given Quantity

I want to be able to calculate a price based on a given quantity. But here is the trick.

Example: Widget comes in cases of 12 at $1.00 per case. If the customer orders 36 widgets, then the cost is 36 x $1.00.

If item A is ordered in quantities less than "12" than there is a 15% upcharge.
Example: Widget comes in cases of 12 at $1.00 per case. If the customer orders 4 widgets then the price is 4 x $1.15

But, if the customer orders 35. Then the first 24 are calculated at 24 x $1.00 and the remaining 11 are calculated at 11 x $1.15.