A retail store sells three types of products, P, Q, and R. The store's fixed costs are $90,000 per month. The variable costs per unit for product P, product Q, and product R are $12, $18, and $24 respectively. If the selling prices per unit for product P, product Q, and product R are $40, $50, and $60 respectively, how many units of each product must the store sell to maximize profit?