The distance between the library and the school is 4.2 kilometers. After 20 minutes from school to the library, Xiao Wei rode a bicycle, and he walked 5/7 of the distance. This is how much Xiao Wei opened. The answer is 3 kilometers.

The following is an introduction to the application questions:

Application problem is to describe relevant facts in language or words and reflect some mathematical relationship (such as quantitative relationship, positional relationship, etc.) ) and solved an unknown number of problems. Each application problem includes known conditions and problems.

In the past, China's application problems usually required narrative to meet three requirements: no contradiction, that is, conditions and problems could not be contradictory; Completeness, that is, the condition must be sufficient to ensure that the value of the unknown quantity can be obtained from the condition; Independence, that is, several known conditions cannot be deduced from each other.

Mathematics application problems in primary schools are usually divided into two categories: simple application problems that can be solved in one step only by addition, subtraction, multiplication and division; Complex application problems that need two or more operations to solve.

For the above information, please refer to Baidu Encyclopedia-Application Questions.