$a)\frac{1}{2}+0,4=\frac{5}{10}+0,4=0,5+0,4=0,9$  

$0,65-\frac{1}{4}=0,65-0,25=0,40=0,4$ $0,65-\frac{1}{4}=0,65-0,25=0,40=0,4$  

$1,75+\frac{5}{6}=1\frac{75}{100}+\frac{5}{6}=1\frac{3}{4}+\frac{5}{6}=1\frac{9}{12}+\frac{10}{12}=1\frac{19}{12}=2\frac{7}{12}$  

 

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

$1,2:\frac{4}{5}=\frac{12}{10}\cdot \frac{5}{4}=$   $\frac{{\sout{6}}^3}{{\sout{5}}^1}\cdot \frac{{\sout{5}}^1}{{\sout{4}}^2}=\frac{3}{2}=1\frac{1}{2}$  

$0,5:\frac{1}{8}=\frac{5}{10}:\frac{1}{8}=\frac{1}{{\sout{2}}^1}\cdot \frac{{\sout{8}}^4}{1}=\frac{4}{1}=4$