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Help with arithmetic mean

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by Dinesh K121 » Wed Oct 26, 2016 10:56 pm
What is the average (arithmetic mean) of the first 90 measurements in a certain list of 92 measurements?

(1) The average of 92 measurements is 7.0 centimeters.

(2) The average of the last 2 measurements is 7.2 centimeters.


I selected C as the answer but it is incorrect according to the solution given. Correct answer is A; I do not have the explanation. Can anyone explain me how A is the correct choice?
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by crackverbal » Thu Oct 27, 2016 2:33 am
Hi Dinesh K121,

The correct answer here is C.

Statement 1: The average of 92 measurements is 7.0 centimeters

To know the average of 90 measurements we need to know the sum / average of the last two measurements (91 and 92). No information about the sum / average is provided. Statement 1 is insufficient

Statement 2: The average of the last 2 measurements is 7.2 centimeters

This statement does not give us any information about sum / average of the first 90 measurements. Statement 2 is insufficient

Combining the two statements

Sum of 92 measurements = 92*7 [from statement 1]
Sum of Last 2 measurements = (7.2)*2 [from statement 2]

Sum of First 90 Measurements = 92*7 - (7.2)*2

Since we know the sum of the first 90 measurements and the number of measurements, we can find the average of the first 90 measurements. Sufficient

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by [email protected] » Thu Oct 27, 2016 11:06 am
Hi Dinesh K121,

You are correct - the correct answer IS C. What is the source of this question (and why does it not offer an explanation?)? There's a decent chance that the answer that the source gave you is just an error/typo, but a resource that doesn't provide an explanation along with the question might not be the best resource to be using during your studies.

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by Dinesh K121 » Sat Oct 29, 2016 5:32 am
Hi Rich, thank you for your answer. The question is from a preparatory test and I cant disclose the details here.
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by ceilidh.erickson » Mon Oct 31, 2016 7:34 am
Dinesh K121 wrote:Hi Rich, thank you for your answer. The question is from a preparatory test and I cant disclose the details here.
My advice to you would obviously be to stop studying from this source: it is clearly not a carefully written one, and would thus not give you any useful information about whether you're likely to do well on test day.

This is an open forum - in the spirit of collaboration, we're all here to help each other do well. Part of that is helping other students to assess which resources are high-quality and which are not. That's why the standard practice is to post your sources.
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