A retail store sells four types of products, P, Q, R, and S. The store's fixed costs are $70,000 per month. The variable costs per unit for product P, product Q, product R, and product S are $10, $15, $20, and $25 respectively. If the selling prices per unit for product P, product Q, product R, and product S are $30, $40, $50, and $60 respectively, how many units of each product must the store sell to maximize profit?