The Waterfall Buffet in the lower level of the National Art Gallery serves self-service buffet daily from 9:00 am to 5:00 pm. Customers arrive at a rate of 60 per hour from 7:00 am to 9:00 am; a rate of 40 per hour from 9:00 am to noon, at a rate of 70 per hour from noon to 2:00 pm; and at a rate of 110 per hour from 2:00 pm to 5:00 pm. All customers take about the same amount of time to serve themselves from the buffet. Once a customer finishes the buffet, it takes 1.5 minute to pay the cashier. The Waterfall Buffet does not want a customer to wait more than 3 minutes to pay. How many cashiers should be working at each interval during the day?