$a)$  

$\frac{1}{2}+0,4=0,5+0,4=0,9$    

$0,65-\frac{1}{4}=0,65-0,25=0,4$  

$1,75+\frac{5}{6}=1\frac{3}{4}+\frac{5}{6}=1\frac{9}{12}+\frac{10}{12}=1\frac{19}{12}=2\frac{7}{12}$  

 

$b)$  

$\frac{5}{4}\cdot 0,2=\frac{{\sout{5}}^1}{{\sout{4}}^2}\cdot \frac{{\sout{2}}^1}{{\sout{10}}^2}=\frac{1}{4}$  

$1,2:\frac{4}{5}=\frac{12}{10}:\frac{4}{5}=\frac{{\sout{12}}^3}{{\sout{10}}^2}\cdot \frac{{\sout{5}}^1}{{\sout{4}}^1}=\frac{3}{2}=1\frac{1}{2}$   

$0,5:\frac{1}{8}=\frac{1}{2}:\frac{1}{8}=\frac{1}{{\sout{2}}^{\cdot }}\frac{{\sout{8}}^4}{1}=4$