12) 4 1/4 : (2 7/10 - 3 1/8)

12) \(4\frac{1}{4} : (2\frac{7}{10} - 3\frac{1}{8}) = \frac{17}{4} : (\frac{27}{10} - \frac{25}{8}) = \frac{17}{4} : (\frac{108}{40} - \frac{125}{40}) = \frac{17}{4} : (\frac{-17}{40}) = \frac{17}{4} \cdot \frac{-40}{17} = -10\)
Ответ: -10