Is the name of a question, which mainly means that there are two different objects together, but I don't know how many. But we know the total number of two objects (similar to how many heads a chicken and a rabbit have) and the total number of different parts of two objects (similar to the total number of feet of a chicken and a rabbit). There is a way to solve this problem. Take chickens and rabbits in the same cage as an example. First, consider the total number of two animals as one animal. Here, consider the total number of chickens first and figure out how many feet there are. Because the chicken's feet are different from the rabbit's feet, there will be differences. The number of feet calculated from the number of feet given in the question. Divided by the difference between the number of feet of a chicken and a rabbit, it is the number of rabbits. The total number of rabbits is the number of chickens.