Describe a red-black tree on $n$ keys that realizes the largest possible ratio of red internal nodes to black internal nodes. What is this ratio? What tree has the smallest possible ratio, and what is the ratio?
The math can get quite complicated if $n$ is not one less than a power of $2$. Reasoning informally:
The smallest possible ratio is obtained by black-only nodes and is going to be $0$.
The largest possible ratio is obtained by each black node having two red children, and is going to be $2$. Here's an illustration:
There are 5 black internal nodes and 10 red internal nodes, making a ratio of $2$.