Consider a regression situation in which y = hardness of molded plastic

Consider a regression situation in which y = hardness of molded plastic and x = amount of time elapsed since termination of the molding process. Summary quantities included the following.

n = 16, SSResid = 1236.43, and SSTo = 25,320.368.

(a) Calculate a point estimate of σ. (Give the answer to three decimal places.) 

On how many degrees of freedom is the estimate based? 

(b) What proportion of observed variation in hardness can be explained by the simple linear regression model relationship between hardness and elapsed time? (Give the answer to three decimal places.)