At the beginning of the season, 1000 fish were tagged. During the subsequent fishing, in the total catch of 5

At the beginning of the season, 1000 fish were tagged. During the subsequent fishing, in the total catch of 5 thousand fish, 350 tagged were found. What was the population size before the start of the fishing?

There were 14,286 of them. This is easy to see if you make up the simplest proportion. So, we take for the unknown, that is, for X, the total number of the population – actually what is asked about in the problem. Opposite X we put 1000 – this is the number of marked fish. Further, already in the top line, we put in front of 5000 (the number of fish caught) the figure 350 – the number of marked ones, of those that were caught.
By multiplying 5000 and 1000, and dividing this product by 350, we get the indicated number (rounded off).