Determine the optimum preventive maintenance frequency for each of the pieces of equipment if breakdown time is normally distributed: $($Round "Probability Ratio" to $4$ decimal places, and all other answers to $2$ decimal places. Negative values should be indicated by a minus sign.$)$Equipment Average Time $($days$)$between Breakdowns Standard Deviation

A $201203$

B $400303$

C $850455$

Equipment Preventive

Maintenance Cost Breakdown

Cost

A$201$ $$330$ $$2,230$

B$400$ $$230$ $$3,530$

C$850$ $$580$ $$4,825$