I'm pretty sure it would be $19. You can check my work to make sure.
First, I multiplied 15 by 20%, which in decimal form would be .20 or .2. I got the answer 4.00, which could also be simplified to 4. Afterwards I added 4 to 15 to add the profit the market gets when they have to pay back the $15 for pound. 4+15=19, so $19 for 20%.