Some Field Experience in the use of an Accelerated Method in Estimating 28-Day Strength of Concrete" considered regressing y =28-day standard-cured strength (psi) against x =accelerated strength (psi). Suppose the equation of the true regression line is y =1800 +1.3x.

a. What is the expected value of 28-day strength when accelerated strength= 2500? (2 marks)
b. By how much can we expect 28-day strength to change when accelerated strength increases by 1 psi? (4 marks)
c. Answer part (b) for an increase of 100 psi. (2 marks)
d. Answer part (b) for a decrease of 100 ps