OE

This is a weighted average question. Since the numbers of workers in each category are given in percentages, pick 100 as the number of workers in each city. Because you only need to find the difference between the two weighted averages, there are shortcuts that greatly simplify the calculations. In the “Less than high school” category, there would be 10% of 100 or 10 workers in C and there would be just 5 in D. Those 5 “extra” workers in C earn 5 × $500 = $2,500 weekly. There are the same number of high school graduates in both cities, so there will be no difference. Move along to the “Some college” group: there are 30 in C and 25 in D. The weekly earnings for 5 such people is 5 × $750 = $3,750. In both of these categories of workers, the total earnings in City D are less than those in City C. Finally, there are 10 more of the high-earners in City D. Those people make 10 × $1,200 = $12,000 per week. So, the total weekly earnings in City D are $12,000 − $3,750 − $2,500 = $5,750 greater than in City C. Remember that this comparison was based upon 100 workers, so the difference in the averages is 5,750/100=$ 57,50 which is (B). If you didn’t use the shortcut and actually computed the weighted averages, they are $921.25 and $863.75, which gives the same $57.50 difference.