Differents ways of ordering bars and stars

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SUMMARY

The discussion focuses on the combinatorial problem of ordering bars and stars, specifically using the "stars and bars" theorem. The participant calculates the total arrangements by fixing one bar and counting the remaining positions, leading to a total of 28 arrangements. The conversation also explores the concept of distinguishability in arrangements, questioning how many distinct orderings exist when elements are indistinguishable. The key takeaway is the application of combinations, specifically "8 choose 2," to determine the number of ways to arrange distinguishable and indistinguishable elements.

PREREQUISITES
  • Understanding of combinatorial principles, specifically the "stars and bars" theorem.
  • Familiarity with the concept of distinguishability in permutations and combinations.
  • Basic knowledge of binomial coefficients, particularly "n choose k" notation.
  • Ability to perform calculations involving combinations and arrangements.
NEXT STEPS
  • Study the "stars and bars" theorem in combinatorics for deeper insights.
  • Learn about binomial coefficients and their applications in combinatorial problems.
  • Explore distinguishability in permutations and how it affects counting arrangements.
  • Practice solving combinatorial problems involving indistinguishable objects.
USEFUL FOR

Mathematicians, students studying combinatorics, educators teaching probability and statistics, and anyone interested in solving complex arrangement problems.

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Homework Statement
How many different ways can we plop down the stars and bars?
Relevant Equations
...
1599420659862.png

Actually, the answer is
1599420682496.png

But i am not sure why we can apply combination here. I am a little confused.
I could get the answer fixing the bar 1 in each place, with this fixed, we could change the position of the other bar. That would be:
First we have 7 different position
After this, 6 different position
5
.
.
.
0

Sn = 8*(7+0)/2 = 28

But i don't know why this is a combination.
Technically, is not
***|*|** = ||****** in the point of view of combination (The order does not matter)?
 
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You can look at it different ways. You counted possibilities, and the other way is what the book does: Each of the eight places has two possibilities. That is drawing eight times either a ball with a star or with a bar out of the bowl and putting back the drawn ball. So we have eight places and two balls, which is 8 choose 2.
 
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If I affix numbers 1-6 to the stars and 1-2 to the bars, so they are all distinguishable, how many distinct orderings are there?

If I removed the numbers from the bars, for any given layout how many became indistinguishable from it? So how many distinguishable arrangements are there?

If I then removed the numbers from the stars (so neither stars nor bars are distinguishable), for any given layout how many became indistinguishable from it?
 
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The question is equivalent to asking "how many distinct subsets of size 2 can I form from the set ##\{1,2,3,4,5,6,7,8\}##?"
 
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