13.9 The accompanying summary quantities resulted from a study in which x was the number of photocopy machines serviced during a routine service call and y was the total service time (minutes):

n 5 16 o(y 2 y])2 5 22,398.05 osy 2 y⁄ d2 5 2620.57

a. What proportion of observed variation in total service time can be explained by a linear relationship between total service time and the number of machines serviced?

b. Calculate the value of the estimated standard deviation se. What is the number of degrees of freedom associated with this estimate?