Already solved the #4TechChallenge for this month? Here’s a bonus question to keep your mind in challenge mode:

Why do no prime numbers have digits that sum to 15?

Post your methods and solutions in the comments below and look out for the answers next month.

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What's the answer?

For those of you that had a crack at the bonus question our justification is here:


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  • Any number that has digits that sum to 15 must be divisible by 3 (this is because 100, 10 and 1 are all equivalent to 1 mod 3). Therefore any number with digits summing to 15 must be divisible by 3, and so cannot be prime.

    Posted by: Daniel on 16/07/2016

  • Thanks for the reply Daniel!

    Can you provide a bit more detail for your justification? What about if the digits sum to 18 or 21 etc....?

    Posted by: James (Deloitte) on 20/07/2016

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