Keith Shoe Stores carries a basic black dress shoe for men that sells at an approximately

constant rate of $500$ pairs of shoes every three months. Keith$$s current buying policy is to

order $500$ pairs each time an order is placed. It costs Keith $$30$ to place an order. The

annual holding cost rate is $20\%.$ With the order quantity of $500,$ Keith obtains the shoes

at the lowest possible unit cost of $$28$ per pair