3). \( (2 \frac{1}{4} + 4 \frac{5}{6}) : \frac{2}{5} - \frac{3}{4} \)

Решение:

1. \( 2 \frac{1}{4} = \frac{2 \cdot 4 + 1}{4} = \frac{9}{4} \)
2. \( 4 \frac{5}{6} = \frac{4 \cdot 6 + 5}{6} = \frac{29}{6} \)
3. \( \frac{9}{4} + \frac{29}{6} = \frac{9 \cdot 3}{4 \cdot 3} + \frac{29 \cdot 2}{6 \cdot 2} = \frac{27}{12} + \frac{58}{12} = \frac{85}{12} \)
4. \( \frac{85}{12} : \frac{2}{5} = \frac{85}{12} \cdot \frac{5}{2} = \frac{425}{24} \)
5. \( \frac{425}{24} - \frac{3}{4} = \frac{425}{24} - \frac{3 \cdot 6}{4 \cdot 6} = \frac{425}{24} - \frac{18}{24} = \frac{407}{24} \)
6. \( \frac{407}{24} = 16 \frac{23}{24} \)

Ответ: \( 16 \frac{23}{24} \)