<p>Ok Im totally confused on when to use combinations.</p>
<p>Say you have two numbers 4, and 5 then how do I know if I juts multiply 4 x 5 or if I do the permutation thing, where it goes like 4 * 3 * 2 * 1</p>
<p>Like for this question:</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 diffrent assignments of actors are possible?</p>
<p>How do I know weather to do 4 * 4, or weather to do 4 * 3 * 2 * 1? Is there some way to find out?</p>
<p>Any help appreciated, Thanks! :)</p>
<p>you do 4<em>3</em>2*1 - it represents the number of outcomes that are possible</p>
<p>You are supposed to multiply together all the possibilities for each person. So the first person has 4 different choices/possibilities. Second has three cause the first one took a possibility…etc. Then multiply them all together.
= 24</p>
<p>One way to sort this out is to keep asking the question “Now how many choices do I have?” – and then multiply the answers.</p>
<p>So compare the questions:</p>
<ol>
<li> You have 4 blocks: red, blue, yellow and green. In how many ways can you stack them?</li>
</ol>
<p>Let’s see: 4 choices for the bottom, then 3, then 2, then 1 — now multiply 4<em>3</em>2*1</p>
<ol>
<li> You have a large collection of red, blue, yellow and green blocks (many of each). How many ways can you make a stack of 2 blocks?</li>
</ol>
<p>Let’s see: 4 choices for the bottom, 4 for the top — now multiply 4*4</p>
<p>So they are two different seeming problems but the same principle solves them.</p>
<p>Ooh Ok I understand, Thankyou guys :)</p>