54. How many fish are in the pond? A certain ichthyologist wanted to determine how many fish in the pond were suitable for catching. For this purpose, he cast a net with a pre-selected mesh size and, pulling it out, found 30 fish, marked each of them, and threw them back into the pond. The next day, he cast the same net and caught 40 fish, two of which had his marks. How did he approximately calculate the number of fish in the pond based on these data?
54. Let $n$ denote the number of fish in the pond suitable for catching. Then the ratio of the number of marked fish to the total number of fish is $30 / n$.

The second time, the ichthyologist caught 40 fish, two of which were marked. The ratio of the number of marked fish to the total number of fish caught is $1 / 20$.

If we assume that the marked fish are evenly distributed among all the fish in the pond, then both ratios should be the same, i.e.,

$$
\frac{30}{n}=\frac{1}{20}
$$

from which $n=600$.

Therefore, the number of fish in the pond suitable for catching with the given net is approximately 600.