# Calculate the Greatest Common Factor or GCF of 21

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The instructions to find the GCF of 21 are the next:

## 1. Decompose all numbers into prime factors

21 | 3 |

7 | 7 |

1 |

## 2. Write all numbers as the product of its prime factors

Prime factors of 21 | = | 3 . 7 |

## 3. Choose the common prime factors with the lowest exponent

Common prime factors: 3 , 7

Common prime factors with the lowest exponent: 3^{1}, 7^{1}

## 4. Calculate the Greatest Common Factor or GCF

Remember, to find the GCF of several numbers you must multiply the common prime factors with the lowest exponent.

**GCF** = 3^{1}. 7^{1} = 21

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