Consider a B$+$ Tree of order $n($each node can hold up to $n$

Let $n=4$ and we start from an empty tree.

a$)$ What is the minimum number of keys that need to be inserted so that we can

end up with a tree of $3$ levels $($assume root is at level $1).$

b$)$ What is the maximum number of keys a tree of $3$ levels can hold.