Let B for Boys and G for Girls.

Now AM = 90B + 81G = 84(B+G)

We finally get 2B = G i.e. G is twice of B.

G is greater.

B.

There is no need to use algebra here. We just need the weighted average

Girls ----------------------- Average of the class ------------------------ Boys

81 ------------------------------------ 84 ----------------------------------- 90

The distance between 81 and 84 is 3

The distance between 84 and 90 is 6

That means to have the overall average towards (or closer) to the girls they must be more than boys in the classroom. The weight is more.

20-second approach (even less) to solve the question

B is the answer
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