Final answer:
The retailer's markup is 50%.
Step-by-step explanation:
The retailer's markup can be calculated by subtracting the cost price from the selling price and expressing the difference as a percentage of the cost price.
In this case, the cost price is $10 and the selling price is $15. Therefore, the markup is:
Markup = (Selling price - Cost price) / Cost price * 100%
Markup = ($15 - $10) / $10 * 100%
Markup = 50%