How to Solve Permutations and Combinations

How to Solve Permutations and Combinations

Learn how to solve mathematics word problems containing Permutations and Combinations using formulas.

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Step by step guide to solve Permutations and Combinations

  • Permutations: The number of ways to choose a sample of k elements from a set of n distinct objects where order does matter, and replacements are not allowed. For a permutation problem, use this formula:
    \(\color{blue}{_{n}P_{k }= \frac{n!}{(n-k)!}}\)
  • Combination: The number of ways to choose a sample of r elements from a set of n distinct objects where order does not matter, and replacements are not allowed. For a combination problem, use this formula:
    \(\color{blue}{_{ n}C_{r }= \frac{n!}{r! (n-r)!}}\)
  • Factorials are products, indicated by an exclamation mark. For example, \(4!\) Equals: \(4×3×2×1\). Remember that \(0!\) is defined to be equal to \(1\).

Permutations and Combinations – Example 1:

How many ways can first and second place be awarded to \(10\) people?

Solution:

Since the order matters, we need to use permutation formula where n is \(10\) and k is \(2\). Then: \(\frac{n!}{(n-k)!}=\frac{10!}{(10-2)!}=\frac{10!}{8!}=\frac{10×9×8!}{8!}\), remove \(8!\) from both sides of the fraction. Then: \(\frac{10×9×8!}{8!}=10×9=90\)

Permutations and Combinations – Example 2:

How many ways can we pick a team of \(3\) people from a group of \(8\)?

Solution:

Since the order doesn’t matter, we need to use combination formula where n is \(8\) and \(r\) is \(3\). Then: \(\frac{n!}{r! (n-r)!}=\frac{8!}{3! (8-3)!}=\frac{8!}{3! (5)!}=\frac{8×7×6×5!}{3! (5)!}=\frac{8×7×6}{3×2×1}=\frac{336}{6}=56\)

Exercises for Solving Permutations and Combinations

Calculate the value of each.

  1. \(\color{blue}{4!=}\)
  2. \(\color{blue}{4!×3!=}\)
  3. \(\color{blue}{5!=}\)
  4. \(\color{blue}{6!+3!=}\)
  5. There are 7 horses in a race. In how many different orders can the horses finish?
  6. In how many ways can 6 people be arranged in a row?

Download Combinations and Permutations Worksheet

  1. \(\color{blue}{24}\)
  2. \(\color{blue}{144}\)
  3. \(\color{blue}{120}\)
  4. \(\color{blue}{726}\)
  5. \(\color{blue}{5,040}\)
  6. \(\color{blue}{720}\)

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