а)

[tex] \frac{35 {x}^{4}y }{16ab} \times \frac{8 {ab}^{2} }{7 {x}^{3} {y}^{4} } = \frac{5xb}{2y {}^{3} } [/tex]

б)

[tex] \frac{ {y}^{2} - 10y + 25}{ {y}^{2} - 25 } \div \frac{10y - 50}{ {y}^{2} + 5y } = \\ = \frac{(y - 5) {}^{2} }{(y - 5)(y + 5)} \times \frac{y(y + 5)}{10(y - 5)} = \frac{y}{10} = 0.1y[/tex]