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I think this problem is to get the standard deviation of average income of towns.
Firstly, you may refer to the definition of standard deviation in http://www.biox.cn/content/20050818/33539.htm
Following the logic to calculate the standard deviation of the example(the height of students),
you can get the mean(average) income of town respectively, that is income/population. Don't worry to use them.
you use them, and take a correct answer that is the standard deviation of mean income of towns or 乡.
In Stata, you can also use the command "summarize".
summarize variable
You can get the mean and standard deviation of variable which is the mean of towns in that case.
I hope you get a good solution. 
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