Triskaidekaphobia

Number Theory Level 2

While adding the first few prime numbers (2, 3, 5, 7, ... ), I obtained a total value of 225, I then realized that I missed out on counting one number. Which one is missing?


The answer is 13.

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Rajen Kapur by Rajen Kapur , 72, India

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2 solutions

Noel Lo
Apr 28, 2015

I am not sure of the logic behind how to solve this, but I got the clue from the title. Triskaidephobia means fear of the number 13. So sorry. :P

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Henny Lim
Oct 17, 2015

Sum of all primes from 2 to 41 is 238. The sum obtained from the question is 225. Therefore 238 - 225 = 13. This is coherent with question's title. Trisdeka means 13 in Greek.

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