2. The essence of crossover method is a simple form of solving binary linear equations. This kind of problem can also be solved by equations. The specific method of using this rule is as follows: For example, the density of A is 10, the density of B is 8, and the density of their mixture is 9. You can put 9 in the middle, write 10 and 8 on the left, mark AB, and then subtract 9 respectively, and the right is 60. At this time, the ratio is 1: 1.