A survey gathered demographic data from respondents asked them to categorize their annual incomes (in thousands of dollars) in the groups in 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. Compute the approximate average income of the respondents
b. Compute the approximate variance and standard deviation of the incomes of the respondents.