$.$ A store has collected the following information on one of its products:

Demand $=4,500$ units$/$year

Standard deviation of weekly demand $=12$ units

Cycle$-$service level $=90\%($z for $90\%=1\mathrm{.}28)$

Lead$-$time $=2$ weeks

Number of weeks per year $=50$ weeks

a$)$ If a firm uses the continuous review system to control the inventory, what is the average amount of safety stock?

b$)$ How many units should he ordered?