$\text{a)}\frac{2}{5}+0,3+0,8-\frac{1}{2}=0,4+0,3+0,8-0,5=1,5-0,5=1$

$\text{b)}0,9:\frac{1}{5}+\frac{1}{3}-\frac{3}{5}=\frac{9}{{\sout{10}}^2}\cdot \frac{{\sout{5}}^1}{1}+\frac{1}{3}-\frac{3}{5}=\frac{9}{2}+\frac{1}{3}-\frac{3}{5}=4\frac{1}{2}+\frac{1}{3}-\frac{3}{5}=4\frac{15}{30}+\frac{10}{30}-\frac{18}{30}=4\frac{7}{30}$

$\text{c)}1:0,3-2\frac{1}{3}-\frac{5}{6}=1\cdot \frac{10}{3}-2\frac{1}{3}-\frac{5}{6}=3\frac{1}{3}-2\frac{1}{3}-\frac{5}{6}=1-\frac{5}{6}=\frac{1}{6}$

$\text{d)}2\frac{1}{7}\cdot 1,6\cdot 2\frac{1}{3}=\frac{{\sout{15}}^3}{{\sout{7}}^1}\cdot \frac{16}{{\sout{10}}^2}\cdot \frac{{\sout{7}}^1}{3}={\sout{3}}^1\cdot \frac{{\sout{16}}^8}{{\sout{2}}^1}\cdot \frac{1}{{\sout{3}}^1}=8$

$\text{e)}2\frac{6}{11}:\frac{7}{11}:3,4=\frac{{\sout{28}}^4}{{\sout{11}}^1}\cdot \frac{{\sout{11}}^1}{{\sout{7}}^1}:\frac{34}{10}={\sout{4}}^2\cdot \frac{10}{{\sout{34}}^{17}}=\frac{20}{17}=1\frac{3}{17}$

$\text{f)}6\frac{2}{3}\cdot \left(0,25+\frac{1}{5}\right)=\frac{20}{3}\cdot \left(0,25+0,2\right)=\frac{20}{3}\cdot 0,45=\frac{{\sout{20}}^1}{{\sout{3}}^1}\cdot \frac{{\sout{45}}^{15}}{{\sout{100}}^5}=\frac{15}{5}=3$

$\text{g)}1\frac{5}{9}:1\frac{1}{3}\cdot 0,75=\frac{14}{9}:\frac{4}{3}\cdot \frac{3}{4}=\frac{{\sout{14}}^7}{{\sout{9}}^1}\cdot \frac{{\sout{3}}^1}{4}\cdot \frac{{\sout{3}}^1}{{\sout{4}}^2}=\frac{7}{8}$

$\text{h)}3\frac{1}{3}-\frac{1}{3}\cdot 0,6=3\frac{1}{3}-\frac{1}{{\sout{3}}^1}\cdot \frac{{\sout{3}}^1}{5}=3\frac{1}{3}-\frac{1}{5}=3\frac{5}{15}-\frac{3}{15}=3\frac{2}{15}$

$\text{i)}3\frac{4}{9}-1\frac{2}{3}\cdot 1,8=3\frac{4}{9}-\frac{5}{{\sout{3}}^1}\cdot \frac{{\sout{9}}^3}{5}=3\frac{4}{9}-3=\frac{4}{9}$

$\text{j)}4,5\cdot 5\frac{5}{9}+6,4\cdot \frac{3}{8}=\frac{{\sout{45}}^9}{{\sout{10}}^1}\cdot \frac{{\sout{50}}^5}{{\sout{9}}^1}+\frac{{\sout{64}}^8}{10}\cdot \frac{3}{{\sout{8}}^1}=\frac{5}{1}\cdot \frac{5}{1}+\frac{8}{10}\cdot \frac{3}{1}=25+\frac{24}{10}=25+2,4=27,4$