G3.2 In Figure $3, A$ and $B$ are two cones inside a cylindrical tube with length of 20 and diameter of 6 . If the volumes of $A$ and $B$ are in the ratio $3: 1$ and $b$ is the height of the cone $\mathrm{B}$, find the value of $b$.
$\begin{array}{l}\frac{1}{3} \pi \cdot 3^{2}(20-b): \frac{1}{3} \pi \cdot 3^{2} b=3: 1 \\ 20-b=3 b \\ b=5\end{array}$