Suppose that the average time before breakdown is normally distributed with a mean of 2.5 weeks and a standard deviation of .4 weeks. If breakdowns cost an average of $1,200 and preventive maintenance costs $800, what is the optimal maintenance interval? (use a z-score of .43 for your calculation) Every .33 weeks Every 3.25 weeks Every 3.12 weeks Every 2.67 weeks Every .67 weeks