Lift trucks deliver products to a packaging department at a Poisson rate of 125 parts per hour. A unit load is to be designed for the lift truck in order to minimize expected costs. Each trip by the lift truck costs $25. Packaging time is exponentially distributed with a mean of 10 seconds per part using an automatic packaging machine. The cost of parts waiting to be packaged and being packaged is $40 per hour. Determine the optimum size of the unit load.