how to find C.F..??? plzz help

if u have a set of frequencies as under:

f_i C.F.

2 2

5 2 + 5=7

4 7 + 4=11

4 11 + 4=15

6 15 + 6=21

you have to write the first CF as the first frequency itself and then add it to the next frequency to get secong cumulative freq.

then add the second CF to third freq. to get third CF......and so on.

hope it helps!!!