In step deviation we have to assume a mean and then do the calculation for findout the actual mean.

Let assumed mean a = 40

Then D = (x-a)/i

Where i is the class interval.

Here i=10

class     mid point(x)     frequency(f)        D=(x-a)/i        f*D

15-25       20                    4                       -2               -8

25-35       30                   11                      -1               -11

35-45       40                   19                       0                 0

45-55       50                   14                       1                14

55-65       60                    8                        2                16

65-75       70                    2                        3                  6

                                     ` sum f`= 58                             `sumfD` = 17

Mean = a+`sumfD` /`sumf` *i

        = 40+(17/58)*10

        = 42.93

So the mean of the distribution is 42.93