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  1. A natural number n has an even tens digit, if and only if, n leaves a remainder r < 10 when divided for 20.

  2. Find all pairs of prime numbers p and q such that p^q - q^p =1

Diego Loaiza
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    Please change the title. Also, have you searched already here? A full solution can also be found at "artofproblemsolving.com". – Dietrich Burde Jan 21 '22 at 12:42
  • Welcome to MSE. Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or closed. To prevent that, please [edit] the question. This will help you recognize and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers. – José Carlos Santos Jan 21 '22 at 12:42

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