15) (-\frac{1}{4}a³⋅0,4b⁷)³ =

15) ($$\frac{-1}{4} a^3 \cdot 0.4b^7)^3 = (\frac{-1}{4})^3 \cdot (a^3)^3 \cdot (0.4)^3 \cdot (b^7)^3 = \frac{-1}{64} a^9 \cdot 0.064 b^{21} = -0.0015625 a^9b^{21}$$

Ответ: -0.0015625 a⁹b²¹