$\text{a)}\frac{2}{5}=\frac{4}{10}=0,4$

$\frac{9}{10}=0,9$  

$\frac{3}{20}=\frac{15}{100}=0,15$  

$\frac{13}{50}=\frac{26}{100}=0,26$  

$\frac{3}{4}=\frac{75}{100}=0,75$  

$\frac{15}{8}\stackrel{\cdot 125}{=}\frac{1875}{1000}=1,875$  

$\frac{121}{20}=\frac{605}{100}=6,05$  

$\text{b)}\frac{1}{2}=\frac{5}{10}=0,5$

$\frac{8}{5}=\frac{16}{10}=1,6$  

$\frac{1}{8}\stackrel{\cdot 125}{=}\frac{125}{1000}=0,125$  

$\frac{5}{8}\stackrel{\cdot 125}{=}\frac{625}{1000}=0,625$  

$\frac{9}{4}=\frac{225}{100}=2,25$  

$\frac{132}{20}=\frac{66}{10}=6,6$  

$\frac{512}{40}=\frac{128}{10}=12,8$  

$\text{c)}\frac{3}{50}=\frac{6}{100}=0,06$

$\frac{17}{100}=0,17$  

$\frac{51}{200}=\frac{255}{1000}=0,255$   

$\frac{9}{500}=\frac{18}{1000}=0,018$  

$\frac{11}{1000}=0,011$  

$\frac{1200}{250}=\frac{120}{25}=\frac{24}{5}\stackrel{\cdot 2}{=}\frac{48}{10}=4,8$