A survey that collected demographic data from respondents asked them to categorize their annual incomes (in thousands of dollars) into the groups in the following table. The number of respondents for each income group is also shown in the table.

Income                        Number of Respondents

$ 20 to under $ 30 …………….          67

$ 30 to under $ 40 ……………          111

$ 40 to under $ 50 ……………          125

$ 50 to under $ 60 ……………..         21

$ 60 to under $ 70 ……………           38

a. Calculate the approximate average income of the respondents.

b. Calculate the approximate variance and standard deviation of the incomes of the respondents.