Question 1039304
Let the class interval <40 be the zero basis for the step deviation.
Note that the class mark for the basis class interval is 35, and the class width is 10.

d   |  f  | df
----------------
-3 | 2  | -6
-2 | 4  | -8
-1 | 29 | -29
0  | 27 | 0
1  | 18 | 18
2  | 20 | 40 
----------------
   | 100 |15

==> Mean  = {{{35+sum(df, i=1,6)/N = 35 + (15/100)*10 = 36.5}}}.