Math CentralQuandaries & Queries


Question from April:

A researcher catches 30 fish in a pond, attaches a marker to their fin and releases them. The next day she places a large net in the water and catches 9 fish. 3 have the marker attached to their fin. Can she predict the number of fish in the pond???
Thank you

Hi April,

This method of predicting the number of fish in the lake assumes that after the 30 marked fish are released they distribute themselves uniformly with the remaining fish in the lake. The next day she catches 9 fish, 3 of which are marked. Hence $\large \frac{3}{9} \normalsize = \large \frac{1}{3}$ of her catch was marked. If the basic assumption is true then $\large \frac{1}{3}$ of the fish in the lake were marked. If 30 fish were marked, how many fish are in the lake?


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