$\text{a)}\frac{1}{2}-\frac{3}{6}=\frac{3}{6}-\frac{3}{6}=0$

$\frac{1}{2}-\frac{3}{8}=\frac{4}{8}-\frac{3}{8}=\frac{1}{8}$

$\frac{4}{5}-\frac{19}{25}=\frac{20}{25}-\frac{19}{25}=\frac{1}{25}$  

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$\text{b)}2\frac{2}{3}-1\frac{4}{5}=2\frac{10}{15}-1\frac{12}{15}=1\frac{25}{15}-1\frac{12}{15}=\frac{13}{15}$

$1\frac{2}{6}-\frac{2}{3}=1\frac{2}{6}-\frac{4}{6}=\frac{8}{6}-\frac{4}{6}=\frac{4}{6}=\frac{2}{3}$

$2\frac{1}{4}-1\frac{6}{8}=2\frac{1}{4}-1\frac{3}{4}=1\frac{5}{4}-1\frac{3}{4}=\frac{2}{4}=\frac{1}{2}$

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$\text{c)}9\frac{1}{2}-3\frac{5}{8}=9\frac{4}{8}-3\frac{5}{8}=8\frac{12}{8}-3\frac{5}{8}=5\frac{7}{8}$

$5\frac{1}{10}-4\frac{3}{5}=5\frac{1}{10}-4\frac{6}{10}=4\frac{11}{10}-4\frac{6}{10}=\frac{5}{10}=\frac{1}{2}$

$2\frac{1}{21}-1\frac{5}{7}=2\frac{1}{21}-1\frac{15}{21}=1\frac{22}{21}-1\frac{15}{21}=\frac{7}{21}=\frac{1}{3}$

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$\text{d)}2\frac{7}{9}-1\frac{1}{6}=2\frac{14}{18}-1\frac{3}{18}=1\frac{11}{18}$

$3\frac{4}{15}-1\frac{11}{30}=3\frac{8}{30}-1\frac{11}{30}=2\frac{38}{30}-1\frac{11}{30}=1\frac{27}{30}=1\frac{9}{10}$

$2\frac{7}{30}-1\frac{17}{20}=2\frac{14}{60}-1\frac{51}{60}=1\frac{74}{60}-1\frac{51}{60}=\frac{23}{60}$