7. Вычислите: \( \frac{3}{10} - \frac{9}{25} : (4-1\frac{9}{10}) + \frac{4}{7} \)

Решение:

1. \( 4-1\frac{9}{10} = 4 - \frac{19}{10} = \frac{40-19}{10} = \frac{21}{10} \)
2. \( \frac{9}{25} : \frac{21}{10} = \frac{9}{25} \cdot \frac{10}{21} = \frac{9 \cdot 10}{25 \cdot 21} = \frac{3 \cdot 2}{5 \cdot 7} = \frac{6}{35} \)
3. \( \frac{3}{10} - \frac{6}{35} = \frac{3 \cdot 7 - 6 \cdot 2}{70} = \frac{21-12}{70} = \frac{9}{70} \)
4. \( \frac{9}{70} + \frac{4}{7} = \frac{9 + 4 \cdot 10}{70} = \frac{9+40}{70} = \frac{49}{70} = \frac{7}{10} \)

Ответ: \(7/10).