5) (-1 7/9)⁷ ⋅ (-1 7/9)¹² : (-1 7/9)²⁰;

5) $$(-1\frac{7}{9})^7 \cdot (-1\frac{7}{9})^{12} : (-1\frac{7}{9})^{20} = (-1\frac{7}{9})^{7+12-20} = (-1\frac{7}{9})^{-1} = (-\frac{16}{9})^{-1} = -\frac{9}{16}$$

Ответ: -9/16