One class of 15 students has an average score of 80. A second class of 5 students has an average score of 90. What is the overall average…

Weight each class average by the number of students. The total points are $15(80)+5(90)=1200+450=1650$, and there are $20$ students, so the overall mean is $\frac{1650}{20}=82.5$, choice F. Choice G ($85$) is the simple average of $80$ and $90$, which treats the classes as equal in size. Since the larger class scored $80$, the combined average must be much closer to $80$ than to $90$. Choices H ($87.5$) and J ($88$) are even closer to $90$, so they cannot be right.