explanation of correct answer needed

Pulling this together...using the same data as the last few problems.... A new logistics analyst has approached you regarding a potential problem. Her analysis indicates the cost of a back order is $70 for our Autographed Footballs, but she is not sure what the appropriate safety stock should be. Given this back order cost, what would you suggest as the appropriate level of safety stock? Football cost Annual Demand Cost per Order Cost to hold the item per year Economic order quantity $200 Lead Time 5000 units Standard Deviation of Forecasted Demand During Lead Time 2 weeks 20 K to Service Level Conversion kN[k] Conversion 10 15 20 30 35