# number of combinations

• May 14th 2008, 03:55 PM
phil2611
number of combinations
Please tell how you arrived there soi understand it, that is the most imporant thing.

How many ways can sierra select 3 movies to rent from the top 10 rentals?
A.720 ways, B.240 C. 90 ways D.120 ways(Hi)
• May 14th 2008, 04:17 PM
arbolis
I think it's 10 chose 3. That is, $\frac{10!}{(10-3)!3!}=120$. To do it, I didn't think a lot. I mean the formula I gave you is very well known and useful in cases like yours here. It means, if you have 10 elements, how many ways are they if you want to pick 3 of them? This is exactly your problem. So I guess you'll get used to it after having done every kind of problems.
More generally, if there is a total of n element and you want to pick up m elements, the total possibilities are $\frac{n!}{(n-m)!m!}$. You should learn this formula and check out your notes, it is probably there and well explained.
• May 14th 2008, 06:28 PM
phil2611
Thank you
• May 14th 2008, 06:29 PM
phil2611
Quote:

Originally Posted by phil2611
Thank you

hello me