My statistics professor is wanting us to rely on our calculators, but I'm having trouble. Here's an example:

According to a report by Scarborough Research, the average monthly household cellular phone bill is $73. Suppose monthly household cell phone bills are normally distributed with a standard deviation of $11.35. What is the probability that a randomly selected monthly cell phone bill is between $80 and $90? The answer, in my book, is .2008, but I'm getting .20161 on my Nspire (using the normCdf function). I know that isn't that much of a difference in answers, but my online homework is picky, not to mention, I know my professor might have similar choices on our exam.

Any help would be appreciated. Thanks. :)