\frac{1}{3}+\frac{5}{2}-\frac{2}{3}=-\frac{1}{3}+\frac{5}{2}=-\frac{2}{6}+\frac{15}{6}=\frac{14}{6}=2\frac{2}{6}=2\frac{1}{3}
\frac{2}{3}*\frac{21}{4}=(skracamy/ "na/krzyz")=\frac{1}{1}*\frac{7}{2}=\frac{7}{2}=3\frac{1}{2}
(\frac{8}{10}+\frac{1}{4})*\frac{2}{3}+1=(\frac{4}{5}+\frac{1}{4})*\frac{2}{3}+1=(\frac{16}{20}+\frac{5}{20})*\frac{2}{3}+1=\frac{21}{20}*\frac{2}{3}+1=\frac{7}{10}*\frac{1}{1}+1=1\frac{7}{10}
1\frac{3}{5}*\frac{2}{7}=\frac{8}{5}*\frac{2}{7}=\frac{16}{35}