a) \(\dfrac{25x^3y}{7z}.\dfrac{28z}{15x^2y^5}\)
\(=\dfrac{25x^3y.28z}{7z.15x^2y^5}\)
\(=\dfrac{700x^3yz}{105x^2y^5z}\)
\(=\dfrac{20x}{3y^4}\)
b) \(\dfrac{x^2+3x+9}{2x+10}.\dfrac{x+5}{x^3+27}\)
\(=\dfrac{\left(x^2+3x+9\right)\left(x+5\right)}{\left(2x+10\right)\left(x^3+27\right)}\)
\(=\dfrac{x^3+3x^2+9x+5x^2+15x+45}{2x^4+54x+10x^3+270}\)
\(=\dfrac{x^3+8x^2+24x+45}{2x^4+10x^3+54x+270}\)
\(\dfrac{25x^3y}{7z}.\dfrac{28z}{15x^2y^5}=\dfrac{\left(25x^3y\right).28z}{7z.\left(15x^2y^5\right)}=\dfrac{10x.4}{y^4}=\dfrac{40x}{y^4}\)