The 10 items i1, i2, ..., i10 may be arranged in a binary search trees as in Fig below. To get i5, the number of comparision needed is 1; for i2, it is 2; for i8 it is 2; for i1 it is 3, and so on. The average is (1+(2+2) +(3+3+3+3)+(4+4+4))/10, i.e., 2.9.