MHB Calculate ways to form a committee of 3 from 8, DIRECTLY WITHOUT ÷?

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SUMMARY

This discussion focuses on calculating the number of ways to form a committee of 3 from 8 individuals without using division. Participants explore the concept of factorials, specifically 3! and its implications in counting combinations. The discussion highlights the method of Constructive Counting, emphasizing that while there are 8 ways to choose the first member and 7 for the second, the challenge lies in understanding how to represent these choices without division. The conclusion is that the direct multiplication of choices leads to the correct understanding of combinations.

PREREQUISITES
  • Understanding of factorial notation (e.g., 3!)
  • Familiarity with Constructive Counting principles
  • Basic combinatorial concepts (e.g., combinations vs. permutations)
  • Knowledge of counting principles in probability
NEXT STEPS
  • Study the principles of combinations in combinatorial mathematics
  • Learn about the implications of factorials in counting problems
  • Explore advanced counting techniques such as the Binomial Theorem
  • Investigate the differences between permutations and combinations in depth
USEFUL FOR

Mathematicians, educators, students in combinatorics, and anyone interested in understanding counting methods without relying on division.

scherz0
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I grok, am NOT asking about, the answers below. Rather, how can I calculate the final answer DIRECTLY, without division? I don't know why my Latex isn't rendering here? Please see https://math.codidact.com/posts/285679.

Orange underline

1. Unquestionably, $\color{#FFA500}{4 \times 3/2} = 3!$ But how can I construe 3! DIRECTLY WITHOUT DIVISION? What does 3! mean?

Here's my surmisal. Blitzstein's solution hints to this calculation, but he didn't write 3! explicitly. You fix the first person. Then of the 3 people left, you can pick the 2nd person in your 2-person committee in 3 ways. Is this correct?

Red underline

2. Unquestionably, ${\color{red}{\dfrac{8 \times 7 \times 6}{3!}}}$ = 8 × 7. But how can I construe 8 × 7 DIRECTLY WITHOUT DIVISION ? What does 8 × 7 mean?

Here's my surmisal. You can pick the 1st committee member in 8 ways, and the 2nd member in 7 ways. But then can't you pick the 3rd member in 6 ways? By this Constructive Counting (p 38 bottom), the answer ought be 8 × 7 × 6? Why isn't there 6?

Problem 4.1:

(b) In how many ways can a 2-person committee be chosen from a group of 4 people (where the order in which we choose the 2 people doesn't matter)?

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David Patrick, Introduction to Counting & Probability (2005), pp 66-7.
 
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I don't understand why you would WANT to avoid dividing! There are 8 ways to choose one person, say "A", then 7 people left to choose from so 7 ways to choose the second person, "B", then 6 ways to choose the third person, "C". There are 8*7*6 ways to choose "ABC" IN THAT ORDER. (There are 8*7*6= 336 ways to choose a "president, vice-president, and treasurer".) But "ABC", "ACB", "BAC", "BCA", "CAB", "CBA" would be the same three people on a "committee".
 

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