\[ \begin{align*} \dfrac{6}{8} \times \dfrac{4}{5} &= \dfrac{6\times 4}{8\times 5} \\ &= \dfrac{24}{40} \\ &= \dfrac{3}{5} \end{align*} \] \[ \begin{align*} \dfrac{9}{12} \times \dfrac{3}{4} &= \dfrac{9\times 3}{12\times 4} \\ &= \dfrac{27}{48} \\ &= \dfrac{9}{16} \end{align*} \] \[ \begin{align*} \dfrac{10}{15} \times \dfrac{9}{6} &= \dfrac{10\times 9}{15\times 6} \\ &= \dfrac{90}{90} \\ &= 1 \end{align*} \] \[ \begin{align*} \dfrac{16}{20} \times \dfrac{25}{8} &= \dfrac{16\times 25}{20\times 8} \\ &= \dfrac{400}{160} \\ &= \dfrac{5}{2} \end{align*} \]