---
title: Combinations and Permutations
---
## Combinations and Permutations

  Let's say you have 9 people competing to place in the top three of a golf tournament. How many different possibilities are there for the top three in the tournament?
  Well, if we pick first place first, we have 9 people to choose from. After that, we would have 8 to choose from for second place, and 7 for third place. To calculate the total, we simply have to multiply them together:
  
  9x8x7=505
  
  This is an example of a permutation. A permutation is the number of different ordered possibilities that can occur in a given situation. A permutation can be with or without repetition, as can a combination. If we say that there is a permutation for n things with r possibilities, the formulae will be:
  
  #####**With Repetition:**
  n^r
  
  #####**Without Repetition:**
  n\!/(n-r)\!
  
  Returning to the problem at the top, what if they were sitting in three identical chairs instead of having rankings? This is an example of a combination. In a combination, order doesn't matter. Therefore, every permutation of the same combination has to be eliminated. This creates two more formulae:
  
  #####**With Repetition:**
  (r+n-1)\!/(r\!(n-1)\!)
  
  #####**Without Repetition:**
  n\!/(r\!(n-r)\!)
  
  
###Sources
  “Combinations and Permutations.” Math is Fun, www.mathsisfun.com/combinatorics/combinations-permutations.html.


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