方法一:$\frac {2}{5}=\frac {14}{35},$$\frac {4}{7}=\frac {20}{35},$$ \frac {2}{5}<\frac {4}{7}$
方法二:$\frac {2}{5}<\frac {1}{2},$$\frac {1}{2}<\frac {4}{7},$$ \frac {2}{5}<\frac {4}{7}$
方法三:$\frac {2}{5}=\frac {4}{10} ,$$\frac {4}{10}<\frac {4}{7},$$ \frac {2}{5}<\frac {4}{7}$
方法四:$\frac {2}{5}=0.4,$$\frac {4}{7}≈0.571,$$ \frac {2}{5}<\frac {4}{7}$