Use the test for divisibility by 3: if the sum of the digits is divisible by 3, ... | Hacker News

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jemfinch on March 8, 2016 | parent | context | favorite | on: Is this prime?

Use the test for divisibility by 3: if the sum of the digits is divisible by 3, the number is divisible by 3. 5+1=6, so 51 is divisible by 3.

noobie on March 8, 2016 [–]

Oh yeah, that's what I meant by saying "up until you add the digits and it's clear of course.". It's just an instant observation I guess.

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