A pond contains 500 fish, 100 of which have been tagged. You catch 50 fish. How many of them would you expect to be tagged?

The tagged fish make up 100 out of 500 fish, or 1/5 of the pond. If the 50 fish you catch reflect that same proportion, you would expect 50 × 1/5 = 10 tagged fish. The correct choice is 10.

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