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The common multiples of 21 and 28 are 84, 168, 252, 336, ... and so on, never ending; so there is no real Highest Common Multiple.

Do you mean:

  1. Highest Common Factor - the highest number which divides both 21 and 28 which is 7; or
  2. Lowest Common Multiple - the lowest number which is a multiple of both 21 and 28 which is 84.
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14y ago

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