4) 72a²b" : (24a7b"); 25y8

4) $$\frac{72a^5b^4}{25y^8} : (24a^7b^9) = \frac{72a^5b^4}{25y^8} \cdot \frac{1}{24a^7b^9} = \frac{72a^5b^4}{25y^8 \cdot 24a^7b^9} = \frac{3 \cdot 24}{25 \cdot 24 \cdot a^2 \cdot b^5 \cdot y^8} = \frac{3}{25a^2b^5y^8}$$

Ответ: $$\frac{3}{25a^2b^5y^8}$$