Let n-ary tree has X nodes, then
X = Internal nodes + Leaf nodes ...................(1)
Also, Leaf nodes = Internal nodes ( n - 1 ) + 1 ..................(2)
So, given X = 25 , 3-ary tree
Leaf nodes = Internal nodes ( 3 - 1) + 1 ...............( using (2) )
=> Leaf nodes = 2 Internal nodes + 1
Putting in eq. (1)
X = Internal nodes + 2 Internal nodes + 1
25 = 3 Internal nodes + 1
=> Internal nodes = 8
=> Leaf nodes = 25 - 8 = 17