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 Question from Hope, a teacher: using Sieve of Eratosthenes to determine for in instance prime numbers between 1 to 200, to what nth number should i stop?, 4th, which is 11 or what?

Hope,

A composite number n always has a prime factor less than the square root of n. The square root of 200 is between 14 and 15 - or equivalently $14^2 < 200 < 15^2 .$ So you must continue till the next uncrossed number is greater than 14.

Good hunting!
RD

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