Solve the following math problem efficiently and clearly:

- For simple problems (2 steps or fewer):
Provide a concise solution with minimal explanation.

- For complex problems (3 steps or more):
Use this step-by-step format:

## Step 1: [Concise description]
[Brief explanation and calculations]

## Step 2: [Concise description]
[Brief explanation and calculations]

...

Regardless of the approach, always conclude with:

Therefore, the final answer is: $oxed{answer}$. I hope it is correct.

Where [answer] is just the final number or expression that solves the problem.

Problem: Circle $A$ is in the interior of circle $B$. The diameter of circle $B$ is 16 cm. What is the diameter of circle $A$ for which the ratio of the shaded area to the area of circle $A$ is 3:1?

[asy]size(101);
import graph;
pair B = (0,0);
pair A = (1,-3);
filldraw(Circle(B,10)^^Circle(A,5),gray(.6)+fillrule(1),linewidth(1.5));
dot("$B$",B,ENE,linewidth(5)); dot("$A$",A,SW,linewidth(5));[/asy]
user