Question 1039303
Let the class interval 27.5 - 32.5 be the zero basis for the step deviation.
Note that the class mark for the basis class interval is 30, and the class width is 5.

d   |  f  | df
----------------
-3  | 4   | -12
-2  | 20  | -40
-1  | 17  | -17
0   | 15  | 0
1   | 2   | 2
2   | 5   | 10 
3   | 5   | 15
4   | 2   | 8
----------------
    | 70  |-34

==> Mean  = {{{30+sum(df, i=1,8)/N = 30 + (-34/70)*5 = 193/7}}}, or about 27.57.