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Bunuel
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By the looks of it, this question might seem difficult because it’s so verbose. But really, I think this is just a moderately difficult question on Weighted average.

The overall average that the student wants to achieve is 90 to obtain a grade A. But a bigger share of this average comes from the average of the first three exams and a smaller share comes from the average of the fourth exam (which is, of course, the marks obtained in the fourth exam).

The average marks of the first three tests is,
\(\frac{80 + 95 + 92}{3}\) = \(\frac{267}{3}\)

60% of this average contributes to the overall average. This means,
\(\frac{3}{5}\) * \(\frac{267}{3}\) = \(\frac{267}{5}\) = 53.4 is the contribution to the overall average.

To reach an overall average of 90, we need another 36.6 to be added to 53.4.

If he scores 90 on the fourth exam, his contribution to the average will be \(\frac{2}{5}\) * 90 = 36 (40% = \(\frac{2}{5}\)). Clearly, 90 is not sufficient to reach an overall average of 90. Answer options A and E can be eliminated.

If he scores 91 on the fourth exam, his contribution to the average will be \(\frac{2}{5}\) * 91 = 36.4, which is also insufficient to reach 90. Options B and D can be eliminated. The only option left is C, this HAS to be the answer.

If he scores 92 on the fourth exam, his contribution to the average will be \(\frac{2}{5}\) * 92 = 36.8. Now,
53.4 + 36.8 = 90.2
This way, we have also mathematically proved that a 92 on the fourth exam will result in an overall average of at least 90.

In hindsight, this has turned out to be a question on solving an equation than testing you on weighted average. But yes, for you to frame that equation, you will have to know that you have to take the weighted average of the two different groups.

Hope this helps!
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Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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