A study by the National Highway Traffic Safety Administration found that driver fatalities rates were highest for the youngest and oldest drivers. The rates per 1000 licensed drivers for every 100 million miles may be approximated by the function ƒ(x) = k(x - 49)⁶ + 0.8, where x is the driver’s age in years and k is the constant 3.8 * 10^-9. Find and interpret the rate of change of the fatality rate when the driver is

(a) 20 years old; 

(b) 60 years old.