As the logistics manager at the "ABC" distribution center, you're facing a challenge. You've been assigned the responsibility of delivering three product skids to a customer within a 90-hour timeframe. Skid one contains cleaning products and requires three hours for preparation, while skid two consists of dry food and needs four hours for preparation. The customer's requirement is that the number of skids one must be at least three times that of skid two. The distributor earns $50 for skid two and $60 for skid one. 

Your goal is to determine the optimal preparation quantities for each type of skid to maximize the profit of "Good folks." Also,  provide the mathematical formulation to address this scenario.