Refer to S4 Price Setting Analysis for this question.

A surfer making and selling surf-boards has fixed costs of $10,000 and his average variable cost is $100 in labour costs and $100 in materials costs. He sells them to a local surf shop for $300 who sells them to customers for $400. The surfer wants to make an annual profit of $20,000 on the business. How many surf boards does he need to sell?