This is a GRE practice question.
If a node in the binary search tree above is to be located by binary tree search, what is the expected number of comparisons required to locate one of the items (nodes) in the tree chosen at random?
My answer was 3 because $n=8$ and $\lg(n)$ comparisons should be made, and $\lg(8) = 3$. But the correct answer is 2.75. Can someone explain the correct answer? Thanks!