4. A grocery chain has 50 refrigerated trailers. Each trailer is used for approximately 2,500 hours per year. A critical part of the refrigerator is a belt that fails after an average of 575 hours with standard deviation of 175 hours (time between failures is Normally distributed). If the belt is replaced during a scheduled maintenance, the cost is $85, whereas an unscheduled replacement costs $285. What is the optimal scheduled replacement interval? LO2L