What is the ratio of the average (arithmetic mean) weight of students

Analyzing the question:
When a question mentions average, it is important to visualize the total at first and see if it helps.

Combined:
We can have one person in A who weighs 60kg, and one person in B who weighs 100 kg. We can also have one person in A who weighs 60 kg, and two people who weigh 90 kg from B. Then we would have different ratios of weight in class A vs class B. Insufficient.

Ans: E

Note on how to come up with these numbers:
The average is fixed at 80. We used one weight of 60 which is -20 compared to the average. Then we need the rest of the weights to have a combined +20 compared to the average to make up for the low weight. Thus we have average+20 if only one weight above 80 and average+10 twice if we have two weights above 80.