Array is [1,2,3,4,5,6,7,8,9,10].
Average number of comparisons depend on how likely each element is to be searched. Suppose each element is equally likely to be searched i.e. every element has 1/10 prob of being searched.
Now searching 5 takes only 1 comparison (it will be found in first trial only).
Then searching 2 or 8 takes 3 comparisons (1 for equality with 5, which was failed, then 1 for less than or greater than, then 1 for equality with 2 or 8).
Then searching 1 or 3 or 6 or 9 takes 5 comparisons (again 1 for less than or greater than, and 1 for equality)
Finally, searching 4 or 7 or 10 takes 7 comparisons.
So average number of comparisons = $\frac{1+2*3+4*5+3*7}{10} = 4.8$.