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Confused with the problem !

by Uva@90 » Fri Aug 30, 2013 11:07 am
What was the combined average attendance at Memorial Stadium for the month of June and July?

1) Average attendance for the month of June was 23,100, and average attendance for the month of July was 25,200.
2) There were 20 games played in the month of June and 22 games played in the month of July at the stadium.

OA: C

I am Confused Why Option A is wrong .....

Thanks in advance.
Regards,
Uva.
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Source: — Data Sufficiency |

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by Mike@Magoosh » Fri Aug 30, 2013 11:34 am
Uva@90 wrote:What was the combined average attendance at Memorial Stadium for the month of June and July?

1) Average attendance for the month of June was 23,100, and average attendance for the month of July was 25,200.
2) There were 20 games played in the month of June and 22 games played in the month of July at the stadium.

OA: C

I am Confused Why Option A is wrong .....

Thanks in advance.
Regards,
Uva.
Uva,
That's a great question, and I am happy to help. :-)

If you have averages for two groups, the only time the total average of each individual will be the simple average of the two group averages is if the two groups are of equal size. If groups are not of equal size, then they have different "weights", and it would be incorrect to count them equally in an average.

Simple example:
Suppose group #1 has 20 points, each with $50. Group average = $50
Suppose group #2 has 5 point, each with $450. Group average = $450

If we take the average of those two numbers, that's $250 ---- that is NOT the average of all 25 people, because the smaller group, Group #2, is overweighted, over-represented, when we average the two groups as if they are the same.
Instead, we would have to add up all the money --(20)(50) + (5)(450) = $3250, and divide by 25 --- 3250/25 = $90. That's the real average, very different from averaging the two group averages.

Choice (A) is a very common and very tempting trap. The more deeply you can understand the nature of this trap, the more likely you are to avoid it in the future.

For more information, see:
https://magoosh.com/gmat/2012/gmat-avera ... -formulas/

Does all this make sense?

Mike :-)
Magoosh GMAT Instructor
https://gmat.magoosh.com/
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by [email protected] » Fri Aug 30, 2013 12:23 pm
Hi Uva,

Mike provided a good explanation for the "issue" behind this question, but I think that I can provide some additional insight.

DS questions may appear a bit vague at first; this prompt talks about average attendance for two months, but DOESN'T start you out with much info (were there events at the Stadium every day? If not, then how many events were there? Etc.). Pay very careful attention to how the 2 Facts are written; they often provide you with the information that you NEED to focus on to properly find a pattern and/or answer the question.

Here, Fact 2 tells us that the Stadium DID NOT have an event every day, so we need to consider THAT concept when dealing with Fact 1.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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by Uva@90 » Fri Aug 30, 2013 5:52 pm
Mike,
That's a cool solution..

Rich,
Thanks for your additional information..
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