6. The life of a certain type of engine is normally distributed with a mean of 10 years, and a standard deviation of 1.5 years. (6a). If one engine is randomly selected, what is the probability that the life of this engine is between 8 and 13 years? (3 points) Hint find the z-score, construct the standard normal distribution density curve, shade your seeking area then find the probability. (6b). If 20 engines are randomly selected, what is the probability that the mean life of these 20 engines is between 8 and 13 years? (2 points) Hint find the z-score, construct the standard normal distribution density curve, shade your seeking area then find the probability