What number should replace the question mark? Explain why.

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Hint 1

Title: Prime numbers turned into a composite number turned into prime numbers.

Arrangement of the squares: It has nothing to do with a rainbow, instead it's a depiction of symmetry. The ROYGBIV colors are just there to let you know the order.


1 Answer 1


I think the answer is



Take the single digit prime numbers and concatenate them - 2357.
Symmetrize this number as indicated by the arch in the diagram - 2357532.
This is the composite number we create.
Now factorise this number as a product of primes $$2357532 = 2 \times 2 \times 3 \times 3 \times 3 \times 83 \times 263$$
Note that this is the ordered factorisation as given in the diagram with 263 missing.

  • $\begingroup$ Nice work, this is it! $\endgroup$ Feb 10 at 14:45

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