If you wish to be specific then read on:
�15 sale price, less �11 cost = �4 "Gross Profit", if you wish to express this as a percentage, you do so by dividing the Gross Profit by the Selling price and multiplying by 100, so 4 divided by 15 x 100 = 26.66% This is known as the "GPM" (Gross Profit Margin) and is represented as a percentage of sales.
Profit is never calculated as a percentage of cost. Trotbots calculation will give you the "Mark up" which is completely different from Profit.