Simplify: $\left(\frac{81}{16}\right)^{-3/4} \times \left(\frac{25}{9}\right)^{-1/2}$.

$\left(\frac{81}{16}\right)^{-3/4} = \left(\frac{16}{81}\right)^{3/4} = \left(\frac{2^4}{3^4}\right)^{3/4} = \frac{2^3}{3^3} = \frac{8}{27}$. $\left(\frac{25}{9}\right)^{-1/2} = \left(\frac{9}{25}\right)^{1/2} = \frac{3}{5}$. Product = $\frac{8}{27} \times \frac{3}{5} = \frac{24}{135} = \frac{8}{45}$.