kanika123 wrote:Last year, I bought an average of 5 CDs a month. For the first 8 months of this year I bought an average of 7 CDs a month. What's the average number of CDs a month I bought during the entire 20-month period?
A. 5.8
B. 7.2
C. 6
D. 7
E. 5

The answer choices allow us to answer this question without performing any calculations.

Notice that this is essentially a weighted averages question.

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

Given:
The person bought 5 CDs per month for 12 months
And, the person bought 7 CDs per month for 8 months


First notice what would have happened IF . . .
The person bought 5 CDs per month for 12 months
And, the person bought 7 CDs per month for 12 months
Since the timeframes are equal, the average over the 24-month period is midway between 5 and 7.
In this hypothetical scenario, the final average is 6 CDs per month.


Of course, the timeframes are not equal. So, one timeframe will influence the final average more than the other timeframe (thus the weighted averages).

Since the person spent MORE TIME buying 5 CDs per month than he/she spent buying 7 CDs per month, the final average will be closer to 5 than it is to 7.
In other words, the final average will be less than 6 (and greater than 5).

So, we can ELIMINATE answer choices B, C, D and E.

This leaves us with answer choice A, the correct answer.

Cheers,
Brent