[ Percentage and ratio problems ]

One time, a fisherman cast a net into a pond and caught 30 fish. Marking each fish, he released them back into the pond. The next day, the fisherman cast the net again and caught 40 fish, two of which were marked. How can we use this data to approximately calculate the number of fish in the pond?

#
It is reasonable to assume that among the caught and uncaught fish in the second round, the marked fish

are distributed equally.

## Solution

If we assume that among the caught and uncaught fish in the second round, the marked fish are distributed equally, then the proportion of marked fish in the pond is $2 / 40 = 1 / 20$. There are a total of 30 marked fish in the pond, so the total number of fish is 20 times greater.