Problem: 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. He has estimated the holding cost per litre to be $\backslash (\backslash$$ $0\mathrm{.}1\backslash )$ per year and the cost of order to be $\backslash (\backslash$$ $5\backslash ).$ Lead time from the producer is two days. The store is open seven days a week. Find optimum order quantity