According to the American Geriatric Society, the maximum recommended heart rate for a 20-year-old man under stress is 200 beast per minute. The maximum recommended heart rate for a 60-year-old man under stress is 160 beats per minute.

(a) Find the linear function that relates that maximum recommended heart rate for men to age.
(b) Find a linear function that relates the maximum recommended heart rate for a 45-year-old man under stress.
(c) Interpret the slope.
(d) For what age would the maximum recommended heart rate under stress be 168 beats per minute?