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Calculate the Greatest Common Factor or GCF of 42, 66, 78 and 126

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The instructions to find the GCF of 42, 66, 78 and 126 are the next:

1. Decompose all numbers into prime factors

422
213
77
1
662
333
1111
1
782
393
1313
1
1262
633
213
77
1

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

Prime factors of 42=2 . 3 . 7
Prime factors of 66=2 . 3 . 11
Prime factors of 78=2 . 3 . 13
Prime factors of 126=2 . 32 . 7

3. Choose the common prime factors with the lowest exponent

Common prime factors: 2 , 3

Common prime factors with the lowest exponent: 21, 31

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 = 21. 31 = 6


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