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If I have a pocket pair, what is the probability that someone else has pockets?

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That depends on the number of opponents there are.
Heads up:
There are 1225 other hole card combinations, and 73 of them are pocket pairs, so slightly worse than 1 in 17. If you do not have a pocket pair, then there are 72 possible pocket pairs out there, so it's slightly (1 in 1225) less likely for your opponent to have a pocket pair.