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Are you sure 51 isn't prime? – Analysing the results of "Is this prime?" (2016)
- anthk 2y ago57 it's 60-3, not prime, clearly. 51 it's 17x3, not so difficult to decompose.
- ColinWright 2y agoThis answer is completely correct, but it makes me feel like you didn't read the linked post.
- beardyw 2y agoJust adding the digits until you get to a single figure divisible by 3 weeds out many of them. 5+7=12, 1+2=3. You are permitted to omit the last step! So 91 had me wondering for a while.