If a number ends in an odd number of zeros, it is

If a number ends in an odd number of zeros, it is
  1. always a perfect square
  2. never a perfect square
  3. sometimes a perfect square
  4. always a cube

Solution

Trailing zeros of a perfect square are always even (since $10^{2k}$ contributes $2k$ zeros). So odd trailing zeros means it cannot be a perfect square.

Asked in: IMO

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