Two partners, 'P' and 'Q,' invested in a business with initial amounts of Rs. 4,800 and Rs. 7,200, respectively. 'P' maintained the investment for 15 months, while 'Q' did so for 12 months. If 'P's share of the profit amounted to Rs. 3,500, what was the combined profit earned by both 'P' and 'Q' in the business?