The cumulative frequency distribution below records the weights of 60 mangoes picked from an orchard.
Weight less than (g): 100 | 120 | 140 | 160 | 180 | 200
Cumulative frequency: 8 | 18 | 33 | 48 | 55 | 60
Convert the data to class frequencies and find the standard deviation of the weights (correct to one decimal place).