Sonal crossed out 1 and every multiple of 2, 3 and 5 except 2, 3 and 5 themselves. One number she has NOT crossed out is still not prime. Which is it?

Question

Accepted Answer

49. 49 = 7 × 7 . Multiples of 7 whose smallest prime factor is 7 are not caught by 2, 3 or 5, so 49 survives. In the sieve you must also cross multiples of 7. (39 is already crossed: 39 = 3 × 13 .)

Sonal crossed out 1 and every multiple of 2, 3 and 5 except 2, 3 and 5 themselves. One number she has NOT crossed out is still not prime. Which is it?

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