Same problem statement:

Weekly demand for DVD$-$Rs at a retailer is normally distributed with a mean of $1,000$ boxes and a standard deviation of $150.$ Currently, the store places orders to the supplier, with a reorder point of $4,200$ boxes. The order quantity to the supplier is fixed at $5,000$ boxes. Replenishment lead time is $4$ weeks, fixed order cost per order is $$100,$ each box costs the retailer $$10,$ and the inventory holding cost is $25\%$ per year. Assume $50$ weeks in a year.

Given the problem parameters, what is the optimal order quantity the retailer should order?