Question on easy math sat I question (nCr/nPr)

Test Preparation
1

<p>had a horrible adv. alegra teacher, didn't teach us anything. Alright, here's one of the questions on a practice test i just took but it doesn't have explanations to the answers</p>

<p>Exactly 4 actors try out for the 4 parts in a play. If each actor can perform any one part and no one will perform more than one part, how many different assignemnts of actors are possible.</p>

<p>My first instinct was nCr/nPr. and it says exactly...which is how u choose which one i think. How do u do this problem. Also, if you do use nCr, how do u put it in on a Ti-89. Thx!</p>

2

<p>nCr. That's it.</p>

3

<p>How do you know when to you use Cr and when to you use Pr?</p>

4

<p>i think it's if it says "exactly" but i'm not sure. we need a math buff in here. Also, how do u put this into Ti-89. because u have to put it in as a fucntion or something.</p>

5

<p>Whoops, I f'ed up. It's a permutation problem, not a combination one. Four different ways of arranging four people.</p>

6

<p>Permutation: 4!= 4<em>3</em>2*1= 24 ways</p>

<p>This problem is as easy as cake, but not quite as tasty IMO.:)</p>

<p>BTW, A permutation is one of the different arrangements of a group of items where order matters.</p>

<p>The number of permutations of n objects taken r at a time is: P(n,r)=n!/(n-r)!</p>

<p>A combination is one of the different arrangements of a group of items where order does not matter.</p>

<p>The number of combinations of a group of n objects taken r at a time is:
n!/r!(n-r)!</p>

7

<p>Yep, that's a better explanation than my lazy one.</p>