Sean has 12 cylindrical containers filled with sand, each of which has a radius of 4 centimeters. He wants to transfer his sand into some new cylindrical containers, which are the same height but have a radius of 8 centimeters. How many new containers will Sean need?
We will need to know how much sand Sean has, so we need to find the volumes of each of the two cylindrical containers. If the height of each container is $h$, the old container holds $4^2h\pi = 16h\pi$ cubic centimeters of sand. The new container holds $8^2h\pi = 64h\pi$ cubic centimeters of sand. We can now find how many new containers Sean will need:  \begin{align*}
&12\text{ old containers}\cdot \frac{16h\pi\text{ cm}^3}{1\text{ old container}} \cdot \frac{1 \text{ new container}}{64h\pi\text{ cm}^3} \\
&\qquad = \boxed{3}\text{ new containers}.
\end{align*}