$($Quantity Discount Model$)$ Keith Shoe Stores carries a basic black dress shoe that sells at a 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 $$30$ to place an order. The annual holding cost rate is $20\%.$ With the order quantity of $500$ pairs, Keith obtains the shoes at the lowest possible cost of $$28\mathrm{.}00$ per pair. Other quantity discounts ordered by the manufacturer are described as follows:Order Quantity Price per pair$0-99$ $$36100-199$ $$32200-299$ $$30300$ or more $$28$ What is average flow time using the basic setup $($assuming $250$ working days in a yea$)?$ Side note $31\mathrm{.}25$days and $62\mathrm{.}50$ days are wrong answers.