An item of stock has a unit cost of 40, a reorder cost of 50 and a

Question:

An item of stock has a unit cost of £40, a reorder cost of £50 and a holding cost of £1 a unit a week. Demand for the item has a mean of 100 a week with a standard deviation of 10. Lead time is constant at three weeks.
Design a stock policy that gives the item a service level of 95%. How would the costs change with a 90% service level?