The percentage mark up refers to the amount of money added to the cost price of goods to cover for the overheads and profit. In this case, the mark up price is 35% of the cost price, therefore: (35%=0.35)
Mark up = 0.35 * $15
Mark up = $5.25
However this is not yet the selling price. The selling price is the mark up value plus the original cost price, therefore:
Selling price = $5.25 + $15
Selling price = $20.25
Therefore the product is sold at $20.25.
Alternate: Another way to solve this is to directly multiply the cost price by 135%, that is: (135% = 1.35)
Selling price = 1.35 ($15)
Selling price = $20.25