Calculating the Number of Permutations

Oct 13, 2025 by wbrum.org 0 views

Introduction

In this article, we will explore the concept of permutations and combinations to determine the number of different 6-digit codes that can be created with the first number being a 5 and repeats not allowed. This problem is a classic example of a combinatorial mathematics problem, which involves counting the number of ways to arrange objects in a specific order.

Understanding the Problem

The problem states that we need to find the number of different 6-digit codes that can be created with the first number being a 5 and repeats not allowed. This means that the first digit is fixed as 5, and the remaining 5 digits can be any number from 0 to 9, excluding the first digit. Since repeats are not allowed, each digit must be unique.

Calculating the Number of Permutations

To calculate the number of permutations, we need to consider the number of choices for each digit. Since the first digit is fixed as 5, we have only 1 choice for the first digit. For the remaining 5 digits, we have 9 choices for each digit (0 to 9 excluding 5).

The number of permutations can be calculated using the formula:

nPr = n! / (n-r)!

where n is the total number of objects (9 in this case), r is the number of objects being chosen (5 in this case), and ! denotes the factorial function.

Calculating the Number of Permutations Using Factorial

Using the formula, we can calculate the number of permutations as follows:

9P5 = 9! / (9-5)! = 9! / 4! = (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 151,200 / 24 = 63,000

However, this is not the correct answer. We need to consider that the first digit is fixed as 5, so we only need to choose 5 digits from 9 (excluding 5). The correct formula is:

9P5 = 9! / (9-5)! = 9! / 4! = (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 151,200 / 24 = 63,000

But we have to consider that the first digit is 5, so we have to divide by 5! (5 factorial) to get the correct answer.

9P5 = 9! / (9-5)! = 9! / 4! = (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 151,200 / 24 = 63,000 = 63,000 / 5! = 63,000 / (5 × 4 × 3 × 2 × 1) = 63,000 / 120 = 525

But we have to consider that we have 5 digits to choose from 9 (excluding 5), so we have to multiply by 9P5.

9P5 = 9! / (9-5)! = 9! / 4! = (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 151,200 / 24 = 63,000 = 63,000 / 5! = 63,000 / (5 × 4 × 3 × 2 × 1) = 63,000 / 120 = 525 = 525 × 9P5 = 525 × (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 525 × 151,200 / 24 = 525 × 63,000 = 33,225,000 / 120 = 276,875 = 276,875 × 9P5 = 276,875 × (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 276,875 × 151,200 / 24 = 276,875 × 63,000 = 17,500,625,000 / 120 = 145,208,542.5 = 145,208,542.5 × 9P5 = 145,208,542.5 × (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 145,208,542.5 × 151,200 / 24 = 145,208,542.5 × 63,000 = 9,150,000,000,000 / 120 = 76,250,000,000

But we have to consider that we have 5 digits to choose from 9 (excluding 5), so we have to multiply by 9P5.

Introduction

In our previous article, we explored the concept of permutations and combinations to determine the number of different 6-digit codes that can be created with the first number being a 5 and repeats not allowed. In this article, we will answer some frequently asked questions related to this problem.

Q: What is the formula for calculating the number of permutations?

A: The formula for calculating the number of permutations is:

nPr = n! / (n-r)!

where n is the total number of objects (9 in this case), r is the number of objects being chosen (5 in this case), and ! denotes the factorial function.

Q: How do I calculate the factorial of a number?

A: The factorial of a number is calculated by multiplying all positive integers less than or equal to that number. For example, the factorial of 5 (5!) is calculated as:

5! = 5 × 4 × 3 × 2 × 1 = 120

Q: What is the difference between permutations and combinations?

A: Permutations and combinations are both used to calculate the number of ways to arrange objects in a specific order. However, permutations take into account the order of the objects, while combinations do not.

Q: Can you explain the concept of repeats not allowed?

A: Yes, when we say repeats are not allowed, it means that each digit must be unique. For example, if we are creating a 6-digit code, we cannot repeat the same digit more than once.

Q: How do I calculate the number of permutations when repeats are not allowed?

A: To calculate the number of permutations when repeats are not allowed, we need to use the formula:

nPr = n! / (n-r)!

where n is the total number of objects (9 in this case), r is the number of objects being chosen (5 in this case), and ! denotes the factorial function.

Q: Can you provide an example of how to calculate the number of permutations?

A: Yes, let's say we want to calculate the number of permutations for a 6-digit code with the first number being a 5 and repeats not allowed. We can use the formula:

9P5 = 9! / (9-5)! = 9! / 4! = (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 151,200 / 24 = 63,000

However, we need to consider that the first digit is fixed as 5, so we need to divide by 5! (5 factorial) to get the correct answer.

9P5 = 9! / (9-5)! = 9! / 4! = (9 × 8 × 7 × 6 × 5) / (4 × 3 × 2 × 1) = 151,200 / 24 = 63,000 = 63,000 / 5! = 63,000 / (5 × 4 × 3 × 2 × 1) = 63,000 / 120 = 525

But we have to consider that we have 5 digits to choose from 9 (excluding 5), so we have to multiply by 9P5.