а) $$ \frac{5y}{7d} : \frac{4c}{3x} = \frac{5y}{7d} \cdot \frac{3x}{4c} = \frac{5y \cdot 3x}{7d \cdot 4c} = \frac{15xy}{28cd} $$ б) $$ \frac{24a^6x^4}{1} : \frac{16a^3x^7}{1} = \frac{24a^6x^4}{1} \cdot \frac{1}{16a^3x^7} = \frac{24a^6x^4}{16a^3x^7} = \frac{3a^3}{2x^3} $$
Ответ: а) $$\frac{15xy}{28cd}$$; б) $$\frac{3a^3}{2x^3}$$