21. Consider the regression model E(Y) = a + BG + BR+B(GR), where Y = income (thousands of dollars), G= gender (G=1 for men and G = 0 for women), and R = race (R=1 for whites and R == 0 for blacks).

a) Suppose that, in the population. B = 0. Interpret B, and 5

b) The prediction equation for a certain sample is = 16+2G-3R+8GR. By finding the four predicted means for this equation, show that the coefficient 8 of the interaction term is the amount by which the mean for one of the four groups must increase or decrease for the interaction to disappear.