Yep, the answer is ((1 - N^(h-1)) / ( 1 - N)) + M

where,
N: N-ary tree
h: Height of N-ary tree
M: No of leaf nodes

Example:

Suppose we are having 3-nary tree as shown below:

(h:1)                                                                                 (1)

(h:2)                                         ((2)                          (3)                    (4))

(h:3)                ((5)                   (6)            (7))   ((8)  (9) (10))  ((11)  (12)  (13))

(h:4)    ((14)  (15)  (16))  ((17))     <-------- M (i.e. 4) Leaf Nodes

Here,
N: 3-ary tree
h: 4, Height of 3-ary tree
M: 4, No of leaf nodes

Now, Apply the formaula:

= ((1 - 3^(4-1)) / ( 1 - 3)) + 4

= 17
Hence Proved.

Solution by:

The Revolutions (Comp Sci RnD)
A Orkut's Community
http://www.orkut.com/Community.aspx?cmm=13552733

Yahoo Group
http://www.yahoogroups.com/group/TheRevolutions