Решение:

1. a) $$ \frac{3}{10} + \frac{4}{10} = \frac{3+4}{10} = \frac{7}{10} $$

   Ответ: $$ \frac{7}{10} $$

2. б) $$ \frac{25}{100} + \frac{30}{100} = \frac{25+30}{100} = \frac{55}{100} = \frac{11}{20} $$

   Ответ: $$ \frac{11}{20} $$

3. в) $$ \frac{9}{20} - \frac{5}{20} = \frac{9-5}{20} = \frac{4}{20} = \frac{1}{5} $$

   Ответ: $$ \frac{1}{5} $$

4. г) $$ \frac{7}{10} - \frac{1}{4} = \frac{7 \cdot 2}{10 \cdot 2} - \frac{1 \cdot 5}{4 \cdot 5} = \frac{14}{20} - \frac{5}{20} = \frac{14-5}{20} = \frac{9}{20} $$

   Ответ: $$ \frac{9}{20} $$

5. д) $$ \frac{2}{5} + \frac{3}{8} = \frac{2 \cdot 8}{5 \cdot 8} + \frac{3 \cdot 5}{8 \cdot 5} = \frac{16}{40} + \frac{15}{40} = \frac{16+15}{40} = \frac{31}{40} $$

   Ответ: $$ \frac{31}{40} $$

6. e) $$ \frac{6}{8} - \frac{1}{4} = \frac{6}{8} - \frac{1 \cdot 2}{4 \cdot 2} = \frac{6}{8} - \frac{2}{8} = \frac{6-2}{8} = \frac{4}{8} = \frac{1}{2} $$

   Ответ: $$ \frac{1}{2} $$

7. ж) $$ \frac{10}{12} - \frac{1}{6} = \frac{10}{12} - \frac{1 \cdot 2}{6 \cdot 2} = \frac{10}{12} - \frac{2}{12} = \frac{10-2}{12} = \frac{8}{12} = \frac{2}{3} $$

   Ответ: $$ \frac{2}{3} $$

8. з) $$ \frac{7}{12} + \frac{5}{18} = \frac{7 \cdot 3}{12 \cdot 3} + \frac{5 \cdot 2}{18 \cdot 2} = \frac{21}{36} + \frac{10}{36} = \frac{21+10}{36} = \frac{31}{36} $$

   Ответ: $$ \frac{31}{36} $$