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?