Dear vp_help,

There are no answers offered by the book. I want to sure whether I am right of these question.Would you please check it for me? Many thanks again!

-"1.How many different collections of 4 books can Mary taken on vacation if she has 20 books to choose from?"

20!/4!(20-4)!

-"2.In how many ways can 20 books be arranged on a shelf?"
20!

-"3.At a race,how many different orderings are possible for 1st,2nd,and 3rd place if the race has 20 participants?"
12!/(12-3)!

-"4.How many subcommittees of 4 are possible from committee of 12 people?"

12!/4!(12-4)!

Cathy,

Please see comments inserted in-line below. I have overwritten some of your original answers.

- Veritas Help

-"1.How many different collections of 4 books can Mary taken on vacation if she has 20 books to choose from?"

20!/( 4!(20-4)! ) -- correct




-"2.In how many ways can 20 books be arranged on a shelf?"
20! ... correct


-"3.At a race,how many different orderings are possible for 1st,2nd,and 3rd place if the race has 20 participants?"
20 * 19 * 18 ... correct



-"4.How many subcommittees of 4 are possible from committee of 12 people?"

12!/( 4!(12-4)!) ... correct

Thanks.Your answers always give me the courage to go on!