Which of the following is NOT a perfect square?

Which of the following is NOT a perfect square?
  1. $144$
  2. $196$
  3. $250$
  4. $169$

Solution

$144 = 12^{2}$, $196 = 14^{2}$, $169 = 13^{2}$. But $15^{2} = 225$ and $16^{2} = 256$, so $250$ is not a perfect square.

Asked in: IMO

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