A = #5 target showing up in the hole

B = Dart labelled 'throw' is picked

By definition:

Only one target out of 10 is labelled #5. Only 1 dart out of 52 is labelled 'thrown'.

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Or if you simply thought about the problem. Since you're given the fact that a dart is thrown, all you have to consider is the probability of hitting the #5 target out of 10 possible targets.