Ответ:

  1. Складываем целые и дробные части: \(12\frac{3}{8}+8\frac{1}{6}=20+\frac{9}{24}+\frac{4}{24}=20\frac{13}{24}\).
  2. \(2\frac{13}{15}+6\frac{7}{10}=8+\frac{26}{30}+\frac{21}{30}=8+1\frac{17}{30}=9\frac{17}{30}\).
  3. \(9\frac{8}{21}+4\frac{11}{14}=13+\frac{16}{42}+\frac{33}{42}=13+1\frac{7}{42}=14\frac{1}{6}\).
  4. \(8\frac{5}{9}+7\frac{3}{4}+12\frac{7}{12}=27+\frac{20}{36}+\frac{27}{36}+\frac{21}{36}=27+1\frac{32}{36}=28\frac{8}{9}\).