Help with arithmetic mean

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Dinesh K121: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Wed Oct 26, 2016 9:24 am

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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Source: Beat The GMAT — Data Sufficiency | Wed Oct 26, 2016 10:56 pm

crackverbal: Master | Next Rank: 500 Posts; Posts: 157; Joined: Mon Aug 16, 2010 7:30 pmFollowed by: 3 members; Location: India; Thanked: 65 times

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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[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

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.

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

Contact Rich at [email protected]

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Dinesh K121: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Wed Oct 26, 2016 9:24 am

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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ceilidh.erickson: GMAT Instructor; Posts: 2095; Joined: Tue Dec 04, 2012 3:22 pmFollowed by: 247 members; Thanked: 1443 times

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.

Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education

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