Cargo ships arrive at a dock for unloading at a rate of 4 per day. Unloading time averages 2 days per ship. The cost of a cargo ship being at the dock for unloading is

$10,000 a day. The cost to rent space for unloading a ship is $2,500 a day. Assuming a Poisson process, determine the optimum number of unloading spaces to provide for cargo ships.