Sunray Electric Coop uses a fleet of 20 trucks to service its electric network. The company wants to develop a preventive maintenance schedule for the fleet. The probability of a breakdown in year 1 is zero. For year 2, the breakdown probability is .03, increasing annually by .01 for years 3 through 10. Beyond year 10, the breakdown probability remains constant at .13. The maintenance cost per truck is $200 for a random breakdown and $75 for a scheduled maintenance. (a) Develop the associated decision tree. (b) Determine the optimal period (in months) between successive preventive maintenances.