anhhh wrote:Hi,

Can someone help explain why the answer to the first question is 4000-5000?

https://www.mba.com/the-gmat-exam/gmat-e ... 1?next=yes

The answer that I got was 2000-3000 because the average of the values from 12, 16, 20 and 30 was 900+3000+5100+5600 = 14600/4 = 3650.

What did I do wrong?

Thanks!

The problem is that you didn't pay attention to the weights between the ages you designated.

Take a simple example. Imagine someone, at age 21, decides he's going to take that GMAT once a year until he's 25. At age 21, he scores a 400. At age 25, he scores an 800. Can we say that his GMAT average for the five years is (400+800)/2 = 600? Of course not - we need more info.

If his scores for the five years are 400,400,400,400,800 his average will be weighted towards 400 and will be much less than 600.
If his scores for the five years are 400, 800, 800, 800, 800, his average will be weighted towards 800 and will be much greater than 600.

Notice, in the actual example, that the T Rex weight is going to over 5000 kg for the entire decade between ages 20 and 30. (Whereas the weight is under 2000 kg for a mere three years.) So the average weight for the entire period is going to be pulled towards high end of the range.