Question

Lisa can choose 4 of her 9 friends to come with her family on a camping trip. How many ways can she choose the friends that come with her?

a. 126
b. 756
c. 3024
d. 6561

Answer 1

The number of combinations of 9 friends taken 4 at a time is given by:
$9C4=\frac{9!}{4!5!}=126$
Answer choice a is correct.

Answer 2

The concept behind the problem above is Combination. The arrangement of the people chosen are not important so long as Lisa is able to choose 4 out of her 9 friends. This is 9C4 or combination of 9 taken four.
                                  9C4 = 9! / ((9 - 4)!4!))  = 126
Therefore, Lisa can do the task in 126 ways.