A population of 10 cargo aircraft each produces a profit of $4,000 per 24-hour period when not waiting to be unloaded. The time between arrivals is distributed exponentially with a mean of 144 hours. The unloading time at the ramp is distributed exponentially with a mean of 18 hours. It costs $150 per hour to lease a ramp with this unloading capacity. How many ramps should be leased?