$\begin{align}  & \frac{0.21\times 0.72\times 0.00054}{0.006\times 1.68\times 0.063}=\frac{21\times {{10}^{-2}}\times 72\times {{10}^{-2}}\times 54\times {{10}^{-5}}}{6\times {{10}^{-3}}\times 168\times {{10}^{-2}}\times 63\times {{10}^{-3}}} \\ & =\frac{21\times 72\times 54}{6\times 168\times 63}\times \frac{{{10}^{-2-3-5}}}{{{10}^{-3-2-5}}} \\ & =\frac{81648}{63504}\times \frac{{{10}^{-10}}}{{{10}^{-8}}} \\ & =1.28571429\times {{10}^{-2}} \\ & \approx 0.01286\text{    (4s}\text{.f)} \end{align}$