Understanding the Number of Distinct Permutations of a Multiset: A Case Study with 6 Blue, 4 Green, and 3 Red Batches

When dealing with sequences composed of repeated elements, calculating the number of distinct permutations becomes essential in fields like combinatorics, data science, and algorithm optimization. A classic example is determining how many unique sequences can be formed using multiset batches—such as 6 blue, 4 green, and 3 red batches—totaling 13 batches.

The Problem: Counting Distinct Permutations of a Multiset

Understanding the Context

Given a multiset with repeated items, the total number of distinct permutations is computed using the multinomial coefficient. For our case:

  • Blue (B): 6 units
    - Green (G): 4 units
    - Red (R): 3 units
    - Total: 6 + 4 + 3 = 13 batches

The formula to compute the number of distinct permutations is:

\[
\ ext{Number of permutations} = \frac{13!}{6! \cdot 4! \cdot 3!}
\]

Key Insights

Where:
- \(13!\) is the factorial of the total number of batches, representing all possible arrangements if all elements were unique.
- The denominators \(6!, 4!, 3!\) correct for indistinguishable permutations within each color group—the overcounting that occurs when swapping identical elements.

Why Use the Multinomial Coefficient?

Without accounting for repetitions, computing permutations of 13 objects would yield \(13! = 6,227,020,800\) arrangements—but this overcounts because swapping the 6 identical blue batches produces no new distinct sequence. Dividing by \(6!\), \(4!\), and \(3!\) removes the redundant orderings within each group, giving the true number of unique sequences.

Applying the Formula

Now compute step-by-step:

Final Thoughts

\[
\frac{13!}{6! \cdot 4! \cdot 3!} = \frac{6,227,020,800}{720 \cdot 24 \cdot 6}
\]

Calculate denominator:
\(720 \ imes 24 = 17,280\), then \(17,280 \ imes 6 = 103,680\)

Now divide:
\(6,227,020,800 \div 103,680 = 60,060\)

Final Result

The number of distinct permutations of 6 blue, 4 green, and 3 red batches is:

60,060 unique sequences

Practical Applications

This calculation supports a wide range of real-world applications, including:
- Generating all possible test batch combinations in quality control
- Enumerating permutations in random sampling designs
- Optimizing scheduling and routing when tasks repeat
- Analyzing DNA sequencing data with repeated nucleotides

Conclusion

When working with multiset permutations, the multinomial coefficient provides a precise and efficient way to count distinct arrangements. For 13 batches with multiplicities of 6, 4, and 3, the total number of unique sequences is 60,060—a clear example of how combinatorial math underpins problem-solving across science and engineering disciplines.