$a)\frac{\left|AB\right|}{\left|CD\right|}=\frac{10cm}{30cm}=\frac{10}{30}=\frac{1}{3}$

$b)\frac{\left|AB\right|}{\left|CD\right|}=\frac{12\frac{1}{2}dm}{5dm}=\frac{12\frac{1}{2}}{5}=\frac{\frac{25}{2}}{5}=\frac{25}{2}:5=\frac{5}{2}$ $b)\frac{\left|AB\right|}{\left|CD\right|}=\frac{12\frac{1}{2}dm}{5dm}=\frac{12\frac{1}{2}}{5}=\frac{\frac{25}{2}}{5}=\frac{25}{2}:5=\frac{5}{2}$

$c)\frac{\left|AB\right|}{\left|CD\right|}=\frac{2,1m}{1,4m}=\frac{2,1}{1,4}=\frac{21}{14}=\frac{3}{2}$

$d)\frac{\left|AB\right|}{\left|CD\right|}=\frac{30cm}{5dm}=\frac{30cm}{50cm}=\frac{30}{50}=\frac{3}{5}$

$e)\frac{\left|AB\right|}{\left|CD\right|}=\frac{4,2m}{1,2dm}=\frac{4,2m}{12m}=\frac{4,2}{12}=\frac{42}{120}=\frac{21}{60}=\frac{7}{20}$

$f)\frac{\left|AB\right|}{\left|CD\right|}=\frac{1\frac{3}{4}cm}{35mm}=\frac{1\frac{3}{4}cm}{3,5cm}=\frac{1\frac{3}{4}}{3,5}=\frac{\frac{7}{4}}{3\frac{1}{2}}=\frac{\frac{7}{4}}{\frac{7}{2}}=\frac{7}{4}:\frac{7}{2}=\frac{7}{4}\cdot \frac{2}{7}=\frac{2}{4}=\frac{1}{2}$