3, 6, 9, 12, 15 and 18
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The square numbers between 1 and 20 are 1,4,9 and 16.
All whole numbers between 1 and 400 (=20 square).
The only common factor is 1.
There are (not counting 1) three square numbers between 1 and 20.
The least common factor of any set of numbers is 1.