National Business Machines manufactures x model A fax machines and y model B fax machines. Each model A cost $90 to make, and each model B costs $140. The profits are $30 for each model A and $25 for each model B fax machine. If the total number of fax machines demanded per month does not exceed 2100 and the company has earmarked not more than $600,000/month for manufacturing costs, how many units of each model should National make each month to maximize its monthly profits P in dollars?