$\frac{{{r_3}}}{{{r_5}}}\,\, = \,\,\frac{{0.529 \times \frac{{{{(3)}^2}}}{2}}}{{0.529 \times \frac{{{{(5)}^2}}}{2}}}\,\,\,\,\, \Rightarrow \,\,\frac{{{r_3}}}{{{r_5}}}\,\, = \,\,\frac{9}{{25}}\,\,\,\,\, \Rightarrow \,\,{r_3}:{r_5}\,\, = \,\,9\,\,:\,\,25$