Question

You have 7 old books and need to choose 4 to give away. How many different groups of 4 books can you make?

Answer 1

I get 7*6*5=210, am I close?

Answer 2

7c4 =7*6*4*3 = 504

Answer 3

is a combinations problem, nCr in the calc if i remember right,

Answer 4

I don't understand Raheen's answer - where is the 5?

Answer 5

7c4 gives 35 in my calc...

Answer 6

there is the combinations formula attached

Answer 7

Thanks!

Answer 8

Hi Jenn,

Raheen is not quite right. The key is that once you have chosen a book to give away... you can't choose it again! So, this is a pick without replacement problem. So, N-choose-R will work with some modification.

The equation for N-choose-R without replacement is:
N-Choose-R = N!/((R!)(N-R)!

For this problem, that gives:

7!/((4!)(7-4)!) = 7!/(4!3!) = 35

Hope this helps.

Answer 9

Yes! Thanks!