TeX source:
\begin{array}{rclcr}28\;\text{Kinder}&\widehat{=}& 100\,\% \cr \cr 1\;\text{Kind}&\widehat{=}& \dfrac{100\,\%}{28} \cr \cr 3\;\text{Kinder}&\widehat{=}& \dfrac{3\cdot 100\,\%}{28} &\approx& 10{,}71\,\% \end{array}