Ron finds 9 books at a bookstore that he would like to buy, but he can afford only 5 of them. In how many ways can he make his selection? How many ways can he make his selection if he decides that one of the books is a must?

15,120 for the first question. 1,680 if one of the books is a must

Step-by-step explanation:

When Ron chooses his first book, he has 9 options. After that, when he chooses his second book, he only has 8 choices because he already picked one of them. By his third choice, he only has 7 choices and so on.

We can make an equation to represent this,

9 x 8 x 7 x 6 x 5

Multiplying all of that out gets you 15,120.

The second problem is even easier. Since one of the books is already chosen for him, he starts off only having 8 options. Since he is only choosing four books this time, we are only multiplying four numbers.

8 x 7 x 6 x 5

Multiplying all of that out tells you there are 1,680 ways he can make his selection in the second scenario.