$\begin{array}{ll}
\mathrm{X} \rightleftharpoons \mathrm{Y} & \mathrm{k}_1=1 \
\mathrm{Y} \rightleftharpoons \mathrm{Z} & \mathrm{k}_2=2 \
\mathrm{Z} \rightleftharpoons \omega & \mathrm{k}_3=4 \
\mathrm{X} \rightleftharpoons \omega & \mathrm{k}_1 \cdot \mathrm{k}_2 \cdot \mathrm{k}_3 \
& \mathrm{k}=1 \times 2 \times 4 \
& \mathrm{k}=8
\end{array}$