a. The standard deviation of a normal random variable is 20. If a sample of 225 is selected out of a large population, what is the standard error of the mean? Round your answers to 3 decimal places.
b. The average age of a vehicle registered in the United States is 8 years, or 96 months. If a random sample of 36 vehicles is selected, find the probability that the mean of their age is between 98 and 100 months?Assume the standard deviation for the population is 15. Round your answers to 4 decimal places.