Question

What are the common factors that Greg found?

Answer 1

The answer is:  1, 2, 7, 14 .
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Explanation:
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FIrst, list all the factors of 42 (positive integers):
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1, 42
2, 21, 
3, 14,
6, 7
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     Then list all the factors of 28 (positive integers):
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     1, 28
     2, 14,
     4, 7
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        Now, list all numbers that occur for BOTH the factors of "28" AND the factors of "42"
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        1, 2, 7, 14
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      The answer is:  1, 2, 7, 14 .
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