The answer is already given in the Best Selected Answer. I just want to provide one more way to solve this question.
Many students are confused about what should be the definition of degree for Trees.
When Tree is considered to be an undirected graph, then degree of a node is the number of neighbors of that node.
But when Tree is considered to be Rooted tree then definition of Degree is different. Generally, if nothing is mentioned, For a rooted tree, the definition of Degree of node is the number of children of that node.
(When you terms like "Child, Children, Root, Leaf, Leaves, Internal nodes, binary tree" then think of that tree as Rooted tree and in that case, if Degree definition is explicitly Not given, then consider the above definition for Degree )
So, in a binary tree, there are zero degree, one degree, two degree vertices possible.
Let number of leaves (leaves are 0-degree vertices) = $L$,
Number of one degree vertices = $D_1$,
Number of two degree vertices = $D_2$.
For this definition of Degree, we have :
Number of edges = Sum of degrees
$L + D_1 + D_2 -1 = 2D_2 + D_1 $
$L = D_2 + 1$
$D_2 = L-1$
Answer : 199
In the "Best Selected Answer", Arjun sir should have used "Number of Neighbors" instead of using the word "Degree" so that students do not get confused. But they have defined what they mean by the word "Degree" so it shouldn't create confusion.