In order to know how many fish are in a lake conveniently and quickly, fishermen often use a method called "marking before catching". First, grab some fish at random from the lake, such as 1000, mark each fish and put it back in the lake. After a while, I caught some fish at random from the lake. For example, if you catch 200 fish for the second time and see how many fish are marked, if there are 10 fish marked, the fishermen estimate that there are about 20,000 fish in the lake.

Fishermen think this way: 200 fish have 10 marks. If the fish in the lake are evenly distributed, the probability of each marked fish being caught is 10/200 = 1/20. Suppose there are n fish in the lake, among which 1000 is marked, then the probability of each marked fish being caught should be 1000/n. So there are1000/n =120, that is, n =/kloc-0.

Mathematicians usually refer to the above quantity to measure the possibility of an event as "probability". Probability theory is a branch of mathematics that studies the possibility of such random events, and it is widely used in modern science and technology. "How many fish are there in the lake" is one of the most famous and simple questions in probability theory. For another example, the same probability theory principle is used to check the rejection rate of factory products.