$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}\ $
$方法四:\frac{2}{5}=0.4,\frac{4}{7}≈0.571,\frac{2}{5}<\frac{4}{7}$