Calculate mid-pt. of each CI and denote it by m.
Decide any 1 mid-pt. as assumed mean (A).
Calculate deviations (d) of mid-pt. from A i.e., d=m-A.
Find out common factor (C) from d and calculate d' which is d/C.
Multiply step deviation d' with frequency (f) and obtain the total to get Summation fd'.
Find out total no. of items in a series i.e., Summation f or N.
Apply the following formula,
X= A+ Summation fd'/Summation f * C
Hope i helped :)