Решение:

1. $$\frac{5}{7} + \frac{1}{4} = \frac{5 \cdot 4 + 1 \cdot 7}{7 \cdot 4} = \frac{20 + 7}{28} = \frac{27}{28}$$
2. $$\frac{11}{18} - \frac{4}{9} = \frac{11 - 4 \cdot 2}{18} = \frac{11 - 8}{18} = \frac{3}{18} = \frac{1}{6}$$
3. $$\frac{7}{45} + \frac{9}{20} = \frac{7 \cdot 4 + 9 \cdot 9}{180} = \frac{28 + 81}{180} = \frac{109}{180}$$
4. $$\frac{11}{16} - \frac{5}{12} = \frac{11 \cdot 3 - 5 \cdot 4}{48} = \frac{33 - 20}{48} = \frac{13}{48}$$
5. $$\frac{8}{15} - \frac{4}{9} = \frac{8 \cdot 3 - 4 \cdot 5}{45} = \frac{24 - 20}{45} = \frac{4}{45}$$
6. $$\frac{7}{10} + \frac{7}{15} = \frac{7 \cdot 3 + 7 \cdot 2}{30} = \frac{21 + 14}{30} = \frac{35}{30} = \frac{7}{6} = 1\frac{1}{6}$$