4. Вычислите 2 2/5 : (9/10 − 1 5/14).

Преобразуем смешанные числа:

\(2\frac{2}{5}=\frac{12}{5}\), \(1\frac{5}{14}=\frac{19}{14}\).

\(\frac{9}{10}-\frac{19}{14}=\frac{63}{70}-\frac{95}{70}=-\frac{32}{70}=-\frac{16}{35}\).

\(\frac{12}{5}:\left(-\frac{16}{35}\right)=\frac{12}{5}\cdot\left(-\frac{35}{16}\right)=-\frac{21}{4}=-5.25\).

Ответ: −5,25.