A full N-ary tree has M non-leaf nodes, how many leaf nodes does it have?

M + (N ^ (n-1)) = (1 - (N ^ n)) / (1 - N)

Here (N ^ (n-1)) is the number of leaf-nodes.

Solving for this leads to the answer

Leaf nodes = M * (N - 1) + 1

Suppose you have a 3-ary tree

A
B C D
E F G H I J K L M

So, here M=4 and N=3. So using the formula above,

Leaf nodes = M * (N - 1) + 1 = 4 * (3 - 1) + 1 = 9