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**