Combinatorics Problem: Finding Samples with Non-Conforming Chips

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Discussion Overview

The discussion revolves around a combinatorics problem involving the selection of semiconductor chips, specifically focusing on how to calculate the number of samples containing at least one non-conforming chip from a lot of 140 chips, of which 10 do not conform to requirements. The scope includes mathematical reasoning and problem-solving strategies.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant proposes a method of first selecting one non-conforming chip and then four from the remaining chips, questioning the validity of this approach.
  • Another participant illustrates a simplified version of the problem with fewer chips, pointing out that the initial method leads to double counting of combinations.
  • A third participant emphasizes the distinction between "at least one" and "exactly one," suggesting that the misunderstanding lies in interpreting the problem's requirements.
  • A later reply identifies a different mistake in the initial logic, noting that the participant incorrectly stated the number of chips remaining after selecting one non-conforming chip.

Areas of Agreement / Disagreement

Participants express differing views on the correct approach to solving the problem, with no consensus reached on the initial logic presented. Multiple competing interpretations of the problem's requirements and the counting methods remain unresolved.

Contextual Notes

Participants highlight potential misunderstandings regarding the phrasing of the problem and the implications for combinatorial calculations, but do not resolve these issues definitively.

Ownaginatious
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So here is the problem:

A lot of 140 semiconductor chips is inspected by choosing a sample of five chips. Assume 10 of the chips do not conform to customer requirements.

...

c) How many samples of five contain at least one non-conforming chip?

Now what seems logical to me is first choose 1 of the 10 non-performing and then choose 4 from the remaining 139 chips.

What is wrong with my logic here? I don't get the answer the book gets (130,721,752), and instead get 148, 916, 260.
 
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Suppose we simplify it a bit so we have 2 non-performing, A and B, and 2 performing, C and D, and we are looking at a sample size of 2. Using your method we first pick A from the 2 non-performing, and then 1 of the remaining 3, giving us:
AB
AC
AD
Now we pick B from the two non-performing, and 1 of the the remaining 3, giving us:
BA
BC
BD

Unfortunately we have counted AB twice, first as AB and then as BA. So that's why your way doesn't work.

To solve it correctly you should take the total number of samples, which is C(140,5), and then subtract the ones that don't have any non-performing elements, which is C(130,5).
 
"What is wrong with my logic here? I don't get the answer the book gets (130,721,752), and instead get 148, 916, 260."

The phrase "at least one" does not mean the same thing as "exactly one". The problem involved at least one, you answered as if it were exactly one.
 
statdad said:
"What is wrong with my logic here? I don't get the answer the book gets (130,721,752), and instead get 148, 916, 260."

The phrase "at least one" does not mean the same thing as "exactly one". The problem involved at least one, you answered as if it were exactly one.

If you read closer, he actually made a different mistake. He said "choose 4 from the remaining 139 chips" not "choose 4 from the remaining 130 chips" (which would be the "exactly one" option).
 

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