Final answer:
To find the price with a 15% profit, the correct formula is Cost * (1 + 0.15), which adds the original cost to the percentage gain. Other formulas provided are not appropriate for calculating a price with profit.
Step-by-step explanation:
To calculate the price with a 15% profit margin, you can use the formula: Cost * (1 + Profit%). For a 15% profit, this becomes Cost * (1 + 0.15). This accounts for both the original cost and the additional percentage gain. Just to clarify, other formulas mentioned in the question like Cost + (Cost * Profit%) would also give the correct price with profit, whereas the formulas Cost / (1 - Profit%) and Cost - (Cost * Profit%) would not be used for this purpose.
Using a math trick, we could also say that the percentage change in price plus the percentage change in quantity reflects the approximate change when you are dealing with large numbers, but in your case, it's best to stick with the direct multiplication approach for precision.
The mention of average profit is also relevant in that it is the difference between the price and the average cost. It indicates that if the market price is above the average cost, the average profit and thus the total profit is positive.