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What are 3 common multiples of 7 and 21?

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The "multiples" of a number, N, are the numbers N, 2*N, 3*N, ..., etc.  For example, the multiples of 4 are listed below in the usual order.

4, 8, 12, 16, 20, ..., and so on.

So by "common multiple" one means a number that is a multiple of 2 or more numbers.

Try listing the multiples of 7 and 21, separately.

You will find many numbers that appear in both lists.  These are the common multiples of 7 and 21.

In fact, you will find that every multiple of 21 is also a multiple of 7.  Do you see why?

Hope this helps.  If not, I or another tutor will be glad to further assist you.