891 is a composite number. Its factors are 1, 3, 9, 11, 27, 33, 81, 99, 297, and 891. An easy check for some numbers greater than 3 is by adding their digits. If the sum of the digits is 3 or a multiple of 3, the number is a composite. The digits of 891 add up to 18, which is a multiple of 3.
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