Stock prices. For the 900 trading days from January 2003 through July 2006, the daily closing price of IBM stock (in $) is well modeled by a Normal model with mean $85.60 and standard deviation $6.20. According to this model, what is the probability that on a randomly selected day in this period the stock price closed
a) above $91.80?
b) below $98.00?
c) between $73.20 and $98.00?
d) Which would be more unusual, a day on which the stock price closed above $93 or below $70?