Question

I randomly found a Vespiquen in shaking grass in Pokemon Black 2 and caught it in a quick ball, how rare is that?

Answer 1

Vespiquen has a 5%, or 1/20 encounter rate everywhere it can be found in BW2.
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Vespiquen has a catch rate of 45, which means its unmodified catch chance (full health, standard Poke ball, ignoring critical captures) is 45/765, or 1/17.
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The quick ball multiplies this chance by 5 when used on the first turn, so the catch chance for Vespiquen becomes 5/17.
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Therefore, if you find a random Pokemon in the rustling grass in Route 12 or Lostlorn Forest in BW2 and threw one quick ball at it, then the chance that you catch a Vespiquen in the process is (1/20)*(5/17), which equals 1/68.

Comments

Thank you! I thought it was much rarer than that and I wanted to check.

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I/68 isn't exactly common, so you should still feel good!