11-20 21-30 31-40 41-50 51-60 61-70
5 10 13 4 2 1

this is inclusive series.. first mak it exclusive.. it will b lyk this
10.5-20.5=5
20.5-30.5=10
30.5-40.5=13
40.5-50.5=4
50.5-60.5=2
60.5-70.5=1
formula for calcutaing exclusive series is... upper limit of last class interval(say 20) subtracted from lower limit of nxt clas(say 21).. divided by 2
equation will b lyk this
21-20/2
answer will b 0.5... nw reduce this 0.5 in evry class interval's lower limit.. n add 0.5 in evry class intrval's upper limit