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calculate the fish in pond, we can't use the function of average immediately. Due to the fish is swimming everywhere.
- first, we need to get 50 fish, and flag it.
- second, we wait for a long time and catch fish, then calculate how much the flaged fish in total number of catched fish.
- we also can fish in different part of pond and then get the average of the tagged fish to calculate the total number of fish in pond.
