Rolf's Golf store sells golf balls for $$28$ per dozen. The store's overhead expenses are $27\%$ of cost and the owners require a profit of $21\%$ of cost$.$

a$.$ How much does Rolf's Golf store buy the golf balls for?

b$.$ What is the price needed to cover all the costs and expenses?

c$.$What is the highest rate of markdown at which the store will still break even?

d$.$What markdown rate would price the golf balls at cost$?$