A normal population has a mean of 20 and a standard deviation of 4. a. Compute the z value associated with 25. (Round your answer to 2 decimal places.) Z = ? b. What proportion of the population is between 20 and 25? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places.) Proportion = ? c. What proportion of the population is less than 18? (Round z-score computation to 2 decimal places and your final answer to 4 decimal places.) Proportion = ?