Define the internal path length, I(T), of a tree T to be the sum of the depths of all the internal nodes in T. Likewise, define the external path length, E(T), of a tree T to be the sum of the depths of all the external nodes in T. Show that if T is a binary tree with n internal nodes, then E(T)=I(T)+2n.

Need a different explanation from the answers answered by chegg.