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first post here! I recently participated in a small friendly competition and this was one of the questions:
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A number whose only prime factors are 2, 3, 5 or 7 is called a humble number. The first 20 humble numbers are: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 24, 25, 27, ....