Math Is Fun Forum

Ok, here's the problem on my homework.  There were 1600 robberies last year, and then the problem presents percentages, like, 10% of the time between 100 and 200 dollars worth of goods were stolen - I broke down the percentages into numerical values.  I have the minimum amount of goods stolen (0) and the maximum (2500), but I don't know specifically the amount of robberies within the certain ranges.  Like, I know there were for example, 50 robberies where between 100 and 200 dollars were stolen, but nothing else; and I'm supposed to calculate the mean.  Do I just assign a midpoint for the value, like, 150 would be the designated amount and go from there?  HELP!

Thanks!