
A First Course in Probability (10th Edition)
10th Edition
ISBN: 9780134753119
Author: Sheldon Ross
Publisher: PEARSON
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Transcribed Image Text:**Educational Example: Counting Passwords Containing a Specific Digit**
**Problem Statement:**
Consider the problem "A common type of bike lock requires a six-digit password, and only allows digits 0 through 9. How many passwords are there that contain the digit 9?"
**Student's Reasoning:**
A student reasons: There are 6 possible places to put the 9, and then no restriction. So there are \(10^5\) possible arrangements of the other digits. This means the correct answer is \(6 \cdot 10^5\).
**Multiple Choice Answers:**
- **A.** This solution is incorrect, since the student overcounts passwords containing more than one 1.
- **B.** This solution is incorrect, since, in the first step, the student needs to specify which digit gets the one.
- **C.** This solution is incorrect since there are not \(10 \times 10 \times 10 \times 10 \times 10 \times 10\) arrangements of the digits 0 through 8 in the other three places.
- **D.** This solution is correct.
**Explanation of the Options:**
- **Option A** suggests a misunderstanding about overcounting certain passwords. It assumes a misstep in the counting process, but it doesn't identify a correct counting principle.
- **Option B** points out that the student's reasoning might need clarification in identifying which digit gets the 9, which is not relevant in this context.
- **Option C** indicates a misunderstanding by claiming there are no arrangements of \(10:10:10:10:10\) for the remaining digits. However, this concept does not correctly analyze the problem.
- **Option D** correctly identifies that the student's logic was sound. There are 6 possible positions for the digit 9, and for the remaining positions, there are \(10^5\) possible arrangements of the other digits.
This problem illustrates the importance of understanding combinatorial principles in counting distinct possibilities in password formation.
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