13.48 Equal slopes for car prices? Refer to Exercise 13.41, with yn = predicted selling price of used car and x1 = age of car. When equations are fitted separately for U.S. and foreign cars, we get yn = 23,417 - 1,715x1 for U.S. cars and yn = 15,536 - 557x1 for foreign cars.

a. In allowing the lines to have different slopes, we allow for an ____________ between age and type of car in their effects on the price. (Fill in the correct word.)

b. Predict by how much the price changes for a one-year increase in the age of the car. Do you need to do this separately for each type of car? Explain.

c. Based on the separate prediction equations, find the predicted price of a (i) U.S. and (ii) foreign car that is eight years old.