31. Calculate the standard deviation for the following data :CI F0

This question was asked in Ex 13.1, 18

31. Calculate the standard deviation for the following data :
CI F
0 - 10 5
10 – 20 8
20 – 30 7
30 – 40 12
40 – 50 28
50 – 60 20
60 – 70 10
70 – 80 10