A convenience store sell pre-paid mobile phones. It purchases them for $12 each and uses a mark up rate of 250%.

a. The markup rate is the constant of proportionality in the equation y= kx. Write an equation that can be used to find the amount of markup on the cost x of a phone

b. Use the equation to calculate the markup amount. What is the retail price of a phone?

c. Next week the store is going to reduce the price to $31.50. Calculate what percent the sale price is of the retail price.

d. Based on your answer to part C, what is an equation in the form y = kx that relates the sale price to the retail price x?