13.10 A simple linear regression model was used to describe the relationship between y 5 hardness of molded plastic and x 5 amount of time elapsed since the end of the molding process. Summary quantities included n 5 15, SSResid 5 1235.470, and SSTo 5 25,321.368.

a. Calculate an estimate of s. On how many degrees of freedom is the estimate based?

b. What percentage of observed variation in hardness can be explained by the simple linear regression model relationship between hardness and elapsed time?