\[ \begin{align*} \dfrac{28}{35} \times \dfrac{15}{42} &= \dfrac{28\times 15}{35\times 42} \\ &= \dfrac{420}{1470} \\ &= \dfrac{2}{7} \end{align*} \] \[ \begin{align*} \dfrac{8}{12} \times \dfrac{18}{32} &= \dfrac{144}{384} \\ &= \dfrac{3}{8} \end{align*} \] \[ \begin{align*} \dfrac{45}{60} \times \dfrac{25}{54} &= \dfrac{1125}{3240} \\ &= \dfrac{25}{72} \end{align*} \] \[ \begin{align*} \dfrac{12}{15} \times \dfrac{10}{21} &= \dfrac{120}{315} \\ &= \dfrac{8}{21} \end{align*} \]