can someone explain the math?

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 $200 Annual Demand 5000 units Cost per Order $250 Cost to hold the item per year 40% Economic order quantity 177 ead Time 2 WEEKS 10 15 ou Answered 20 30 orrect