In fact, it is very difficult to count the fish in a fish pond one by one, but there is such a simple method, as you said.

Because these 40 fish have been evenly distributed after entering the fish pond.

For example, there are 400 fish in the fish pond, marked with110, that is, 40 fish. Then, when you catch 60 more fish, according to the average probability, there should be 6 fish marked. Of course, in fact, you will find that there may be five or seven. If the RP is not good, even one is not marked, it is possible, but on average, there are six.

Back to the topic you mentioned, we can sum up a formula:

Number of marked fish/total fish in fish pond = number of marked fish fished out/total fish fished out (= proportion of marked fish to all fish)

Substituting data, 40/ total fish in fish pond = marked fish in fished fish /60.

Therefore, the total number of fish in the fish pond =2400/ the number of marked fish in the fish fished out.