Discrete Structures

The text book solution doesnt make sense

4 Let T be a full binary tree. Let / be the sum of the lengths of the simple paths from the root to the internal vertices. We call I the internal path length. Let E be the sum o lengths of the simple paths from the root to the terminal vertices. We call E th external path length. Prove that if T has n internal vertices, then E-I+2n. (Hint: use induction on n.)