## kcla1996 3 years ago You have checked out b books from the library, where b is a number greater than 5. You plan to read three of the books and return them at the end of this week. In how many ways can you pick the books you will read first, second, and third?

1. jim_thompson5910

Hint: you have 'b' choices for the first book, b-1 choices for the second book and b-2 choices for the third book

2. kcla1996

i don't get it

3. jim_thompson5910

4. jim_thompson5910

It's not counting as in (1, 2, 3, 4...), it's counting as counting a very large number of items http://www.regentsprep.org/Regents/math/ALGEBRA/APR1/Lcount.htm

5. kcla1996

b1*b2?

6. jim_thompson5910

close, but not quite

7. jim_thompson5910

Say you pick out 10 books, you would have 10 books for the first choice, 9 for the second, and 8 for third So you would have 10*9*8 = 720 choices possible

8. kcla1996

b^3 ?

9. jim_thompson5910

no

10. jim_thompson5910

multiply out b, b-1 and b-2

11. kcla1996

(5b-3)?

12. jim_thompson5910

no

13. jim_thompson5910

b(b-1)(b-2) = ???

14. kcla1996

b^3-3b^2+2b

15. jim_thompson5910

you got it

16. kcla1996

thank you!

17. jim_thompson5910

yw