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Determine the prime factors for both numbers

18 = 2 x 3 x 3

21 = 3 x 7

As 3 is the only factor common to both numbers then 3 is the Lowest Common Factor and the Highest Common Factor.

NOTE : In general questions are asked for the HighestCommon Factor and/or the Least (or Lowest Common Multiple.

In case the LCM is required , then this is the product of the highest power of each prime factor. Hence, LCM = 2 x 32 x 7 = 2 x 9 x 7 = 126

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Q: What is the Lowest common factor of 18 and 21?
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