Problem Solving

gmat_winter wrote:In calvin's chemistry class, her final grade consists of lab work and the final exam. If lab work is 2/3 of the final grade, and she scores 80%, what must she score on the final exam to earn a final grade of 75%?

A)65%
B)68%
C)70%
D)73%
E)75%

Weighted average of groups combined = (group A proportion)(group A average) + (group B proportion)(group B average) + (group C proportion)(group C average) + ...

So, here we have: Final grade = (lab work weighting)(lab work grade) + (final exam weighting)(final exam grade)
Plug in values to get: 75 = (2/3)(80) + (1/3)(x) [x = final exam grade]
Simplify: 75 = 160/3 + x/3
Multiply both sides by 3 to get: 225 = 160 + x
x = 65
Answer: A

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For more information on weighted averages, you can watch this GMAT Prep Now video: https://www.gmatprepnow.com/module/gmat- ... ics?id=805

Here are some additional practice questions related to weighted averages:
- https://www.beatthegmat.com/weighted-ave ... 17237.html
- https://www.beatthegmat.com/weighted-ave ... 14506.html
- https://www.beatthegmat.com/average-weig ... 57853.html
- https://www.beatthegmat.com/averages-que ... 87118.html

Cheers,
Brent

Hi gmat_winter,

Since the question asks what Calvin's score needs to be on her final exam, and the answers are scores, we can TEST THE ANSWERS.

From the prompt, we know a number of facts:
1) The overall grade is based on 2 "parts" lab work and 1 "part" final exam
2) The lab score was 80%
3) The overall grade is supposed to be 75%

Since the lab score counts "double" and is ABOVE the final grade, the final exam score should be far BELOW the final grade (75%).

Let's TEST Answer B first....

IF.....
Final exam = 68%
Overall grade = [2(80) + 68]/3 = 228/3 = 76
This is TOO HIGH (it's supposed to be 75)

To lower the score, we need a lower final exam grade. There's only one answer that fits....

Final Answer: A

GMAT assassins aren't born, they're made,
Rich