The dimensions of a cylindrical side riser (height = diameter) for a $25 \: cm \times 15 \: cm \times 5 \: cm$ steel casting are to be determined. For the tabulated shape factor values given below, the diameter of the riser (in $cm$) is ________

$$\begin{array}{|l|l|l|l|l|l|l|} \hline \text{Shape factor} & 2 & 4 & 6 &8 & 10 & 12 \\ \hline \text{Riser volume/Casting volume} & 1.0 & 0.70 & 0.55 & 0.50 & 0.40 & 0.35 \\ \hline \end{array}$$