The Average Time Before Breakdown of a Machine Is Normally

Multiple Choice

The average time before breakdown of a machine is normally distributed and has a mean of nine weeks and a standard deviation of 1.5 weeks. If breakdown cost averages $1,500 and preventive maintenance costs $500, what is the optimal preventive maintenance interval?

A) 3.8 weeks
B) 8.4 weeks
C) 9.6 weeks
D) 6.9 weeks
E) 7.2 weeks