One class of 20 students has an average test score of 85. A second class of 30 students has an average score of 90. What is the combined…

Averages of groups must be weighted by group size. Total points are $20(85)+30(90)=1700+2700=4400$, and there are $50$ students, so the combined mean is $\frac{4400}{50}=88$, choice C. Choice B ($87.5$) averages the two class averages, $\frac{85+90}{2}$, which treats the classes as equal in size. Choice A ($87$) is a similar unweighted estimate. Choice D ($90$) is just the larger class's average. Since the second class has more students, the combined mean must lie closer to $90$ than to $85$, so it must exceed $87.5$.