The way you work this out is by thinking about the odds of each singular event, then finding the overall odds based on the individual odds.
The number of different books Mrs. Reid can choose is 9, so the first number is 9.
Mrs. Reid has picked one book so far, so now she has (1 - 9) = 8 books to choose from.
The of different books Mrs. Reid can choose now is 8, so your second number is 8.
Mrs. Reid has picked 2 book thus far, so now there are (2 - 9) = 7 books to choose from.
The of different books Mrs. Reid can choose now is 7, so your second number is 7.
To get the total number of different choices, multiply all the singular events together:
\(7*8*9=504\ different\ ways\ to\ bring\ 3\ books\)
