TeX source:
\begin{array}{rclcl} 100\,\% &\widehat{=}& 500 \,\text{kg} \cr\cr 1 \,\% &\widehat{=}& \dfrac{500}{100} \;\text{kg} \cr\cr 90 \,\% &\widehat{=}& \dfrac{90 \cdot 500 \;\text{kg}}{100} &=& 450 \;\text{kg} \end{array}