TeX source:
\begin{array}{rclcl} 2{,}5 \,\% &\widehat{=}& 16 \;\text{g} \cr\cr 1 \,\% &\widehat{=}& \dfrac{16}{2{,}5} \;\text{g} \cr\cr 100 \,\% &\widehat{=}& \dfrac{16 \cdot 100\;\text{g}}{2{,}5} &=& 640 \;\text{g} \end{array}