The serum zinc level X in micrograms per deciliter for males between ages 15 and 17 has a distribution that is approximately normal with μ = 90 and α = 15. 

a) What is the 90th percentile of X? What does the value you obtained mean? 

b) What is the probability that a randomly selected male in this age category has a serum zinc level above 114, if you already know that his serum level is above 99?

c) Assume 10 males between ages 15 and 17 are randomly selected. What is the probability that at least three of the randomly selected males have a serum zinc level of at least 99?