Rocky Mountain Tire Center sells $20,000$ go$-$cart tires per year. The ordering cost for each order is S$40,$ and the holding cost is $20\%$ of the purchase price of the tires per year.

The purchase price is $$20$ per tire if fewer than $500$ tires are ordered, $$18$ per tire if $500$ or more$$but fewer than $1,000$ tires are ordered, and $$17$ per tire if $1,000$ or more tires are ordered.

a$)$ How many tires should Rocky Mountain order each time it places an order?

b$)$ What is the total cost of this policy?