. Let x represent the number of hours a student spent studying for a test and let y represent the score they received on their exam. The following four results were obtained: (3, 64), (4, 72), (4, 76), (5, 80).

a) Find the least-squares regression line y = mx + b (or y = a + bx), and graph the regression line along with the original data points.

b) Predict the test grade for a student who studied 6 hours.

c) What percent of the variation in test grades are explained by hours of study?

d) What percent are explained by other factors? Name at least one other possible factor.