Final answer:
To find the retail price of the hat, add the markup percentage to the original cost.
Step-by-step explanation:
To find the retail price of the hat, we need to add the markup percentage to the original cost. The markup percentage is calculated by multiplying the original cost by the percentage increase, which is 15% in this case. Therefore, the markup amount is $18 * 0.15 = $2.70. To find the retail price, we add the markup amount to the original cost: $18 + $2.70 = $20.70.
Learn more about calculating retail price with markup