The following table gives information on the amounts (in dollars) of electric bills for August 2015 for a sample of 50 families.

Amount of Electric Bill                                            Number of Families
(dollars)
0 to less than 60....................................................................5
60 to less than 120 .............................................................16
120 to less than 180 ...........................................................11
180 to less than 240 ...........................................................10
240 to less than 300 ..............................................................8

Find the mean, variance, and standard deviation. Give a brief interpretation of the values in the column labeled mf in your table of calculations. What does Σmf represent?