Final answer:
The profit per half kilogram of butter is calculated by subtracting production and joint costs from the selling price; with given values, profit calculation includes selling price, processing cost, and allocated joint cost.
Step-by-step explanation:
To find the profit per half kilogram of butter, one must consider the additional processing cost and the selling price. The profit per half kilogram of butter would be the selling price ($21.00 for half a kilogram because $42.00 is the price per kilogram) minus the processing cost ($13.00, which is half of the $26.00 processing cost per kilogram), and then subtract the joint cost allocated to the half kilogram of butter (which is half of the $5.15 joint cost incurred to produce 4 liters of milk). Therefore, the calculation would be $21.00 (selling price) - $13.00 (processing cost) - $2.575 (half of the joint cost) to find the profit per half kilogram of butter.