The following frequency distribution reports the electricity cost for a sample of 50 two-bedroom apartments in Albuquerque, New Mexico, during 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. Estimate the mean cost.
b. Estimate the standard deviation.
c. Use the Empirical Rule to estimate the proportion of costs within two standard deviations of the mean. What are these limits?