The topic comes from the fifth-grade mathematics book of the New Soviet Education Press, which uses equations to solve practical problems, as follows:

I weigh 36 kilograms this year, 2.5 kilograms more than last year. How much did I weigh last year?

The equivalence relation is: this year's weight-last year's weight =2.5 Let last year's weight be x kg, then equation 36-x=2.5 gives x=33.5, and last year's weight is 33.5kg. Of course, the equivalence relation can also be used: last year's weight 12.5 = this year's weight can also be used to get last year's weight.