Which number is a common multiple 12 and 21?

Answer 1

There is really so such thing as a "greatest common multiple." Once you find the least common multiple of a set of numbers, you can keep adding the LCM to itself over and over again. Each new number you get will be a common multiple of your set of numbers, but each new number will always be larger than the previous. This means that you can keep adding while the number approaches infinity and you will still never find a greatest multiple.

The first few common multiples of 7, 12, and 21 are: 84, 168, 252, 336, and 420.

Answer 2

12, 24, 36, 48, 60.

21, 42, 63, 84, 105.

Answer 3

The GCF is 3.

Answer 4

84 and any of its multiples.