6. [20 points] Dan's Auto Detailing business performs two major activities: exterior cleanup, and interior detailing. Based on the size of car and condition, time estimates for six cars on Monday morning are as shown in the accompanying table. Car Number 1 2 3 4 5 6 Exterior 50 45 70 65 45 80 Interior 60 40 20 55 35 75 a. Sequence the cars so that all exterior detailing is done first and total completion time is minimized. What is the sequence? (Hint: This is a two-facility problem) b. Draw a Gantt chart and evaluate the idle time for each activity. What is the total idle time?