vikram4689 wrote:
I can't find anything wrong here but i also know that GMAC is never wrong

Brent@GMATPrepNow wrote:

vikram4689 wrote:https://www.mba.com/the-gmat/nex-gen/sam ... 1?next=yes

Checkout the Q1 in above link. Given answer is 4000-5000yrs. For this to be true sum of ages should be at least 4000*3=12000yrs

Calculating the sum = 900+3000+5300=9200yrs (which is < 12000yrs)

I can't find anything wrong here but i also know that GMAC is never wrong

The question asks us to consider "all integer values of the age from 12 to 30."
In your solution, you are considering the ages 12, 16 and 20 only.

We actually need to find the mass at ages 12, 13, 14, . . . 29, and 30, and then find the average of these masses.

Now, of course the GMAC doesn't want to perform such a tedious calculation. Instead, we need to get a feel for the average.

First notice that from ages 12 to 20, the increases are somewhat constant. In other words, since that curve from points A to C is close to straight, it looks like the T-Rex's mass increases by the approximately the same amount each year. There's a nice rule that deals with the average of a set of values when those values are equally spaced: In a set where the numbers are equally spaced, the mean will equal the median.

So, if we examine only the masses from ages 12 to 20, the average mass will be approximately equal to the median mass. This median mass will occur in the middle, at age 16.

The T-Rex's mass at at 16 is about 3000 kg, so we can conclude that, during the 8 years between ages 12 to 20, the T-Rex's average mass is 3000 kg.

Now notice that, from ages 21 to 30, T-Rex's average mass is about 5500 kg.

At this point, we have a weighted averages question.

Ages 12 to 20: During these 8 years the average mass is about 3000 kg
Ages 21 to 30: During these 10 years the average mass is about 5500 kg

Given this, we can see that the average mass during the entire 18-year period will be closer to 5500 than it is to 3000.

So, the average mass must be greater than 4250 kg (which is halfway between 3000 and 5500).

I cover this concept of weighted averages in the following video: https://www.gmatprepnow.com/module/gmat- ... ics?id=805

Cheers,
Brent

Brent@GMATPrepNow wrote: Having said that, if the official questions are as difficult as the ones floating around, then we're all in the same boat

Brent@GMATPrepNow wrote:

vikram4689 wrote:https://www.mba.com/the-gmat/nex-gen/sam ... 1?next=yes

Checkout the Q1 in above link. Given answer is 4000-5000yrs. For this to be true sum of ages should be at least 4000*3=12000yrs

Calculating the sum = 900+3000+5300=9200yrs (which is < 12000yrs)

I can't find anything wrong here but i also know that GMAC is never wrong

The question asks us to consider "all integer values of the age from 12 to 30."
In your solution, you are considering the ages 12, 16 and 20 only.

We actually need to find the mass at ages 12, 13, 14, . . . 29, and 30, and then find the average of these masses.

Now, of course the GMAC doesn't want to perform such a tedious calculation. Instead, we need to get a feel for the average.

First notice that from ages 12 to 20, the increases are somewhat constant. In other words, since that curve from points A to C is close to straight, it looks like the T-Rex's mass increases by the approximately the same amount each year. There's a nice rule that deals with the average of a set of values when those values are equally spaced: In a set where the numbers are equally spaced, the mean will equal the median.

So, if we examine only the masses from ages 12 to 20, the average mass will be approximately equal to the median mass. This median mass will occur in the middle, at age 16.

The T-Rex's mass at at 16 is about 3000 kg, so we can conclude that, during the 8 years between ages 12 to 20, the T-Rex's average mass is 3000 kg.

Now notice that, from ages 21 to 30, T-Rex's average mass is about 5500 kg.

At this point, we have a weighted averages question.

Ages 12 to 20: During these 8 years the average mass is about 3000 kg
Ages 21 to 30: During these 10 years the average mass is about 5500 kg

Given this, we can see that the average mass during the entire 18-year period will be closer to 5500 than it is to 3000.

So, the average mass must be greater than 4250 kg (which is halfway between 3000 and 5500).

I cover this concept of weighted averages in the following video: https://www.gmatprepnow.com/module/gmat- ... ics?id=805

Cheers,
Brent