Write a linear cost function equation for each of the following conditions. Use y for estimated costs and X for activity of the cost driver.

a. Direct manufacturing labor is $15 per hour.

b. Direct materials cost $25.60 per cubic yard.

c. Utilities have a minimum charge of $500, plus a charge of $0.25 per kilowatt-hour.

d. Machine operating costs include $250,000 of machine depreciation per year, plus $100 of utility costs for each day the machinery is in operation.