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
