Fifty cities provided information on vacancy rates (in percent) for local apartments in the following frequency distribution.

Vacancy Rate (in percent) ......Frequency

0 up to 3 .............. 5

3 up to 6 .............. 5

6 up to 9 ..............10

9 up to 12 .............20

12 up to 15 ............10

a. Calculate the average vacancy rate.

b. Calculate the variance and the standard deviation for this sample.