Demand for vanilla ice cream at a small ice cream shop can be

approximated by a Normal distribution with a mean of $21$ litres per week

and a standard deviation of $3\mathrm{.}5$ litres per week. The ice cream is

purchased from an ice cream producer. The store manager desires a lead

time service level of $90$ percent. Lead time from the producer is two days.

The store is open seven days a week.