Use the following formula: (2^n)-1. E.g., if the depth is 3, the number of nodes is (2^3)-1 = 8-1 = 7. Note that 7 is the maximum number of nodes, not the actual number of nodes. To count the actual nodes you must traverse the tree, updating an accumulator as you go.

If N>1, there are (2N-1) - (2N-1-1), otherwise, 1 nodes in the Nth level of a balanced binary tree.

1

A full binary tree of depth 3 has at least 4 nodes. That is; 1 root, 2 children and at least 1 grandchild. The maximum is 7 nodes (4 grandchildren).

A binary tree with n nodes has exactly n+1 null nodes or Null Branches.

so answer is 21.

MOHAMMAD SAJID