The document discusses calculating mark-ups, cost of sales, and selling prices. It provides examples of how to calculate the selling price given the cost price and mark-up percentage. It also shows how to calculate the cost price when given the selling price and mark-up percentage. The examples demonstrate calculating a 100% mark-up on an iPod sold for double its cost price and a 50% mark-up on an Xbox sold for 50% above its cost price. It also explains how to calculate the cost price of items if the selling price and mark-up percentage are known, such as NBA jerseys with a 100% mark-up or football boots with a 50% mark-up.