Step 1: Recall the formula for radius of Bohr orbit.
According to Bohr’s model, the radius of the \(n^{th}\) orbit is given by
\[
r_n \propto n^2
\]
For hydrogen atom,
\[
r_n=n^2r_1
\]
where \(r_1\) is the radius of the first Bohr orbit.
Step 2: Write the radii of the \(3^{rd}\) and \(6^{th}\) orbits.
For the \(3^{rd}\) orbit,
\[
r_3=3^2r_1=9r_1
\]
For the \(6^{th}\) orbit,
\[
r_6=6^2r_1=36r_1
\]
Step 3: Find the required ratio.
The ratio of radii is
\[
\frac{r_3}{r_6}=\frac{9r_1}{36r_1}
\]
\[
\frac{r_3}{r_6}=\frac{1}{4}
\]
\[
\frac{r_3}{r_6}=0.25
\]
Step 4: Final conclusion.
Therefore, the required ratio is
\[
\boxed{0.25}
\]