Question

Biologists tagged 180 fish in a lake on January 1. On February​ 1, they returned and collected a random sample of 4545 ​fish, 1515 of which had been previously tagged. On the basis of this​ experiment, approximately how many fish does the lake​ have?
Steve
Assuming typos, 1/3 of the fish found in February were tagged.

So, 180 represents 1/3 of the fish in the lake.