Bloomsday Outfitters produces T-shirts for road races. They need to acquire some new stamping machines to produce 30,000 good T-shirts per month. Their plant operates 200 hours per month, but the new machines will be used for T-shirts only 60 percent of the time and the output usually includes 5 percent that are "seconds" and unusable. The stamping operation takes 1 minute per T-shirt, and the stamping machines are expected to have 90 percent efficiency considering adjustments, changeover of patterns, and unavoidable downtime. How many stamping machines are required?