could you please explain me this thank you

You have a mattress store. You're annual demand for a certain type of mattress is 60 units, you estimate it costs $50 a year to hold a mattress interest on the cash out, warehousing costs, etc), costs roughly $25 per order for a mattress, and you anticipate that if you don't have stock of that mattress, you lose 50% of the buyers of the mattress and the mattress would normally be sold for $800 (so it's a $400 backorder cost). What's the acceptable number back-ordered per cycle