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.$)$

Use Table.

Equipment Average Time $($days$)$

between Breakdowns Standard

Deviation

A$201192$

B$400292$

C$850444$

Equipment Preventive

Maintenance Cost Breakdown

Cost

A$201$ $$320$ $$2,220$

B$400$ $$220$ $$3,520$

C$850$ $$570$ $$4,800$