How Do You Calculate Permutations of Repeated Letters?

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Discussion Overview

The discussion revolves around calculating the number of permutations of the letters in the word "LOLLIPOP," specifically addressing the challenge posed by repeated letters. The scope includes mathematical reasoning and combinatorial principles.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant initially suggests using the formula for permutations of all objects, $8P8$, without accounting for repeated letters.
  • Another participant explains that to find the correct number of arrangements, one must consider the repetitions of letters (3 L's, 2 O's, and 2 P's) and suggests a formula involving factorials.
  • A subsequent reply proposes a calculation using the formula $\frac{8!}{3!2!2!}$ and arrives at the result of 1680 permutations.
  • A later response confirms the calculation and provides a simplified expression for clarity.

Areas of Agreement / Disagreement

Participants appear to agree on the method of calculating permutations with repeated letters, with multiple contributions refining the approach. However, the initial misunderstanding regarding the use of $8P8$ indicates a lack of consensus on the starting point.

Contextual Notes

The discussion does not resolve potential misunderstandings about the application of permutations in cases of repeated elements, nor does it clarify the assumptions behind the initial approach.

nickar1172
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Reviewing for finals and got this question wrong:

How many different permutations are there of the letters in the word LOLLIPOP

what I did was 8P8, how would you solve this?
 
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The number of ways to order or arrange $n$ objects is $n!$. So, we want to look at the number of ways to order 8 letters, however, there are 3 L's, 2 O's and 2 P's. Hence, you want to take the total number of ways to arrange 8 letters, and then account for the fact that some of them are identical. Can you state how many would be identical?

edit: I have removed the [SOLVED] label from the title so that our readers don't skip the thread thinking you have already found the solution yet.
 
so it would be 8P8/2!3!2! = 1680?
 
Yes, although I would simply write:

$$N=\frac{8!}{3!2!2!}=8\cdot7\cdot6\cdot5=1680$$
 

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