11. 5/12 + 3/8; 12. 4/7 − 2/5; 13. 1/3 + 1/4 + 1/6; 14. 5/6 − 3/8 + 1/4; 15. 4/9 + 5/12 − 1/6; 16. 7/10 − 2/15 + 1/5; 17. 2/5 + 3/7 − 1/2; 18. 9/14 − 1/4 + 3/7; 19. 5/9 + 7/12 − 3/4; 20. 11/15 − 3/10 + 2/5.

Приводим дроби в каждом выражении к наименьшему общему знаменателю и выполняем действия по порядку.

1. \(\frac{5}{12}+\frac{3}{8}=\frac{10}{24}+\frac{9}{24}=\frac{19}{24}\).
2. \(\frac{4}{7}-\frac{2}{5}=\frac{20}{35}-\frac{14}{35}=\frac{6}{35}\).
3. \(\frac{1}{3}+\frac{1}{4}+\frac{1}{6}=\frac{4}{12}+\frac{3}{12}+\frac{2}{12}=\frac{9}{12}=\frac{3}{4}\).
4. \(\frac{5}{6}-\frac{3}{8}+\frac{1}{4}=\frac{20}{24}-\frac{9}{24}+\frac{6}{24}=\frac{17}{24}\).
5. \(\frac{4}{9}+\frac{5}{12}-\frac{1}{6}=\frac{16}{36}+\frac{15}{36}-\frac{6}{36}=\frac{25}{36}\).
6. \(\frac{7}{10}-\frac{2}{15}+\frac{1}{5}=\frac{21}{30}-\frac{4}{30}+\frac{6}{30}=\frac{23}{30}\).
7. \(\frac{2}{5}+\frac{3}{7}-\frac{1}{2}=\frac{28}{70}+\frac{30}{70}-\frac{35}{70}=\frac{23}{70}\).
8. \(\frac{9}{14}-\frac{1}{4}+\frac{3}{7}=\frac{18}{28}-\frac{7}{28}+\frac{12}{28}=\frac{23}{28}\).
9. \(\frac{5}{9}+\frac{7}{12}-\frac{3}{4}=\frac{20}{36}+\frac{21}{36}-\frac{27}{36}=\frac{14}{36}=\frac{7}{18}\).
10. \(\frac{11}{15}-\frac{3}{10}+\frac{2}{5}=\frac{22}{30}-\frac{9}{30}+\frac{12}{30}=\frac{25}{30}=\frac{5}{6}\).