$\begin{array}{l}a)\ \dfrac{1,28}{3,15}\\=\dfrac{1,28.100}{3,15.100}\\=\dfrac{128}{315}\\\,\\b)\ \dfrac25\div3\dfrac14\\=\dfrac25\div\dfrac{13}4\\=\dfrac{2.4}{5.13}\\=\dfrac8{65}\\\,\\c)\ 1\dfrac37\div1,24\\=\dfrac{10}7\div\dfrac{31}{25}\\=\dfrac{10.25}{7.31}\\=\dfrac{250}{217}\\\,\\d)\ \dfrac{2\dfrac15}{3\dfrac17}\\=\dfrac{\dfrac{11}5}{\dfrac{22}7}\\=\dfrac{11}5\div\dfrac{22}7\\=\dfrac{11.7}{5.22}\\=\dfrac{7}{10} \end{array}$
