\to  \\ 
 \begin{array}{*{20}c}
   {7h} & {Xh}  \\
\end{array} \\ 
 \begin{array}{*{20}c}
   {15d} & {12d}  \\
\end{array} \\ 
  \leftarrow  \\ 
 7:X = 12:15 \to X = \frac{{7 \cdot 15}}{{12}} \\