$O B=12 \mathrm{~cm}, r=10 \mathrm{~cm}$. Express the surface area of the solid in terms of $\pi$.
The surface area $=2 \pi r^{2}+\pi r L=320 \pi \mathrm{cm}^{2}$
Volume $=\frac{2}{3} \pi r^{3}=\frac{2000 \pi}{3} \mathrm{~cm}^{3}$
Volume of space $=\pi r^{2} h-\frac{1}{3} \pi r^{2} h$
$$
\begin{array}{c}
=\frac{2}{3} \pi r^{2} h \\
\text { Ratio }=\pi r^{2} h: \frac{1}{3} \pi r^{2} h=3: 1
\end{array}
$$
G7.1 Figure 1 shows a cone and a hemisphere.
G7.2 What is the volume of the hemisphere shown in figure 1 ? Give your answer in terms of $\pi$.
G7.3 In figure 2, a right circular cone stands inside a right cylinder of same base radius $r$ and height $h$. Express the volume of the space between them in terms of $r$ and $h$.

G7.4 Find the ratio of the volume of the cylinder to that of the cone.