计算：$\dfrac{7}{25} \div 1\dfrac{2}{5} \div 2\dfrac{1}{4}$

1. 将式中的带分数化为假分数

$1\dfrac{2}{5} = \dfrac{7}{5}, \quad 2\dfrac{1}{4} = \dfrac{9}{4}$

2. 将分数的除法转化为乘法并约分计算

$\begin{aligned} \dfrac{7}{25} \div 1\dfrac{2}{5} \div 2\dfrac{1}{4} &= \dfrac{7}{25} \div \dfrac{7}{5} \div \dfrac{9}{4} \\ &= \dfrac{7}{25} \times \dfrac{5}{7} \times \dfrac{4}{9} \\ &= \dfrac{1}{5} \times \dfrac{4}{9} \\ &= \dfrac{4}{45} \end{aligned}$