The following frequency distribution reports electricity cost for sample of 50 twobedroom apartments in Albuquerque, New Mexico throughout the month of May last year.
Electricity Cost

Frequency

$ 80 up to $100

3

100 up to 120

8

120 up to 140

12

140 up to 160

16

160 up to 180

7

180 up to 200

4

Total

50

a) Determine the mean cost.
b) Determine the standard deviation.
c) Use the Empirical Rule to estimate the proportion of costs in two standard deviations of mean. What are these limits?