Serum potassium concentrations were studied in adults. Serum potassium concentrations had a mean of 4.41 mEq/L and a standard deviation of 0.50 mEq/L. A person was classified as hypokalemic if the serum potassium concentration was below 3.50 mEq/L, "normal" if the serum potassium concentration fell between 3.50 mEq/L and 5.50 mEq/L, and hyperkalemic if the serum potassium concentration was above 5.50 mEq/L. Assuming that serum potassium concentrations follow a normal distribution, answer the following questions.
(a) What is the percentile of a person with a serum potassium concentration of 4.05 mEq/L?
(b) What serum potassium concentration corresponds to the 91st percentile?
(c) What proportion of adults are predicted to fall into the hypokalemic, normal, and hyperkalemic ranges?