$\text{a)}3,6\cdot 2\frac{1}{2}-\frac{1}{2}:\left(\frac{1}{4}-\frac{1}{8}\right)=\frac{36}{10}\cdot \frac{5}{2}-\frac{1}{2}\left(\frac{2}{8}-\frac{1}{8}\right)=\frac{18}{5}\cdot \frac{5}{2}-\frac{1}{2}:\frac{1}{8}=$   

$=\frac{{\cancel{18}}^9}{{\cancel{5}}_1}\cdot \frac{{\cancel{5}}^1}{{\cancel{2}}_1}-\frac{1}{{\cancel{2}}_1}\cdot {\cancel{8}}^4=9-4=5$  

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$\text{b)}\frac{2\frac{1}{3}-3\frac{1}{4}}{3-0,25}=\frac{2\frac{4}{12}-3\frac{3}{12}}{3-\frac{1}{4}}=\frac{-\frac{11}{12}}{2\frac{3}{4}}=\frac{-\frac{11}{12}}{\frac{11}{4}}=-\frac{{\cancel{11}}^1}{{\cancel{12}}_3}\cdot \frac{{\cancel{4}}^1}{{\cancel{11}}_1}=-\frac{1}{3}$