Three formulas are equal to 1, and 9,7 require that their two numbers must be odd and even. This version still has 1 odd 1 even.

However, the formula with the weight result equal to 2 requires both numbers to be odd or even, which cannot meet the above conditions.

Find a routine method:

Numbering: To find a regular topic, we usually give a series of quantities in a certain order, and ask us to find a general rule according to these known quantities. Find out the rule, usually the serial number of the package. Therefore, it is easier to find the mystery by comparing variables with serial numbers.