4) Выполнить действия: a) $$\frac{2}{5} + \frac{3}{10}$$ б) $$\frac{7}{8} - \frac{5}{6}$$ в) $$\frac{12}{45} \cdot \frac{25}{16}$$

a) $$\frac{2}{5} + \frac{3}{10} = \frac{2 \cdot 2}{5 \cdot 2} + \frac{3}{10} = \frac{4}{10} + \frac{3}{10} = \frac{4+3}{10} = \frac{7}{10}$$ б) $$\frac{7}{8} - \frac{5}{6} = \frac{7 \cdot 3}{8 \cdot 3} - \frac{5 \cdot 4}{6 \cdot 4} = \frac{21}{24} - \frac{20}{24} = \frac{21-20}{24} = \frac{1}{24}$$ в) $$\frac{12}{45} \cdot \frac{25}{16} = \frac{12 \cdot 25}{45 \cdot 16} = \frac{3 \cdot 4 \cdot 5 \cdot 5}{5 \cdot 9 \cdot 4 \cdot 4} = \frac{3 \cdot 5}{9 \cdot 4} = \frac{15}{36} = \frac{5}{12}$$