12) $$rac{9}{12}-\frac{1}{3} \cdot \frac{9}{10}: \frac{3}{4}$$

Решим пример по действиям:

1) $$\frac{1}{3} \cdot \frac{9}{10} = \frac{1 \cdot 9}{3 \cdot 10} = \frac{9}{30} = \frac{3}{10}$$

2) $$\frac{3}{10}: \frac{3}{4} = \frac{3}{10} \cdot \frac{4}{3} = \frac{3 \cdot 4}{10 \cdot 3} = \frac{12}{30} = \frac{2}{5}$$

3) $$\frac{9}{12} - \frac{2}{5} = \frac{3}{4} - \frac{2}{5} = \frac{3 \cdot 5 - 2 \cdot 4}{20} = \frac{15 - 8}{20} = \frac{7}{20}$$

Ответ: $$\frac{7}{20}$$