Over the last 10 years, there has been a dramatic increase in the

can somebody please explain the OA?

As per Average concept, if a sample with n elements have average X and for a n+1 element to come and increase the average of the sample, the (n+1)th element need to have value greater than sample average i.e X.

How come increasing the number of people under age 18 increases the average from 52 to 57?

Posted from my mobile device

Hello Subhashish

I'll try to help
we have to strengthen the fact that the number of people over 65 have increased.
Now, how can we increase the average. you are write with your approach. whenever something greater than the average is added to the set.

Now consider this,
we add a figure to a set that is actually less than the average.
Average is going to decrease.
but what if we have to increase the average ?
we will first have to add the relevant amount( equal to the amount less than the average to bring it back to the normal) and then increase it some more.

Answer choice A does exactly just that: its says that the no of people below 18 has decreased. but we already know that the overall average has increased. thus there must be some addition of a figure that is greater than the average to increase the average.

However this choice will make sure that the no of people over 55 has increased, but it can not guarantee that the no of people over 65 has increased.
but if you will look at the other answer choices, they have graver loopholes in them.

Hope this helps.

Kaczet,i’ll do my best to let u understand why A is the answer and not B(since u’ve said B should be the answer)

Conclusion = “the number of people with age over 65 yrs has increased DRAMATICALLY”

Premise = “the average age has increased from 52 to 57”.The premises are facts that need to be taken as hard truths.

A-The number of people under 18 has increased = implies that the number of people older 52 has to increase(because the average has increased)

Now how does this strenthen the conclusion that 65+ increased..?

The difference between (52 and 18) is bigger than (52 and 65).So if the average increased from 52 to 57 the number bigger than 65 has to even bigger because the number below 18 has increased.

B-If the birth rate has come down,it strengthens the fact that the number of 65+ People also should have come down.

Consider this= previously when the birth rate was high(consider their age to be 0 or 1).to even this out(because avg has increased from 52 to 57), the people with 65+ Should have gone up for sure.Now since the number of births has come down,the number of 65+ need not necessarily increase.

Phew!!!excuse me for the long explanation.

Posted from my mobile device

This is a great question.

As per premise:
Cause: Over the last 10 years, the average age of people living in this region has increased from approximately 52 to 57 years.
Effect: there has been a dramatic increase in the number of people over the age of 65 living in this region.

---younger group------Average ------elder group---

The conclusion is strengthened if we can show that age group of over 65 has contributed to the increase in average age from 52 to 57 years, and this is bolstered if the other age group on the left side of average (the younger group) has not pulled the average down.
A indicates just that.

I chose B, but now understand why A is the answer.

For B, if birth rate went down and there is less babies than 10 years ago, this means the average could have just gone up from 52 to 57 if people in other age groups have just aged and increased the average age.

However, for A, teenagers grew steadily, then there must have been a higher increase in the older groups to increase the average age from 52 to 57, therefore strengthening the argument .

Argument - Since, the average age of the people living in a region has increased from 52 to 57 --> number of people over 65 years of age has increased.

(A) The number of people in the region under the age of 18 has increased over the last 10 years.
- Let us consider that the average is given as: (A + B + C) / (number of people). Here A = number of people younger than 52 ; B= number of people in the age range of 52 - 65 ; C = number of people elder than 65
Thus, is A has increased and the average also has increased, hence C (our target group) has to increase
- Correct

(B) The birth rate for the region decreased significantly over the last 10 years.
- Considering the same formula to calculate average --> in this case they are saying that A has decreased, causing the contribution of elders to average age to increase. Thus, the increase is not because of increase in the number of people elder than 65 years old.
- To a certain extent this can infact act as a weakener
- Wrong

(C) The total number of people living in the region has decreased over the last 10 years.
- Similar to B, this can infact act as a weakener.
- wrong

(D) The number of people who moved into the region over the last 10 years is greater than the number of those who moved out.
- This statement explains the rise in the denominator. However, on the numerator side, either of three groups could increase.
- Wrong

(E) The average age for people in the region is higher than that for people in the surrounding regions.
- Irrelevant
- Wrong

Strengthen—#%. The correct answer choice is (A)

The argument in the stimulus is that there has been a dramatic increase in the number of people over the age of 65 in the region in the last 10 years. This conclusion is based on the observation that the average age of people in the region has increased from 52 to 57.

Keep in mind that averages can actually be increased by one of two possible causes: an increase in the number of things above the original average, or a decrease in the number of things below the original average (averages can be lowered by the exact opposite ideas). Here, the author concludes that the average age increase was caused by an increase in the things above the average (more old people), meaning that he/she assumes the other cause—fewer young people—did not occur. Since we are asked to strengthen this causal argument, we need to eliminate the potential alternate cause of the average age increase and show that there are NOT fewer young people in the region now.

Answer choice (A): This is the correct answer choice. As mentioned above, to strengthen a causal argument with two possible causes, you should look for an answer that eliminates the cause you do not want. By stating that there are now more young people, answer choice (A) eliminates the possible cause of fewer young people, and makes it more likely that the cause we want—more old people—is the reason that the average age has gone up. Put another way: if there are now more young people, we would expect the average age to go down. Since it has still gone up, it seems even more likely that a significant number of old people are now in the region, increasing the overall average age. So the conclusion would be strengthened.

Answer choice (B): This is the most frequently chosen incorrect answer, but it actually weakens the argument. As discussed previously, the alternate (and unwanted) explanation for an increase in the average age in the region would be fewer young people, as fewer young people would cause the average age to rise with or without more old people. So if the birth rate has decreased significantly, that means that there are now fewer young people and the average age would be expected to increase. Since this answer choice provides an alternate cause for the average age increase (i.e. not more old people as the stimulus states, but fewer young people instead), it hurts our argument and is incorrect.

Answer choice (C): There is no way to know if a change in the number of people in the region (positively or negatively) would have any impact on the number of people over the age of 65, so this answer choice cannot be thought to have an effect on the argument.

Answer choice (D): Again, an increase in the number of people in the region cannot be known to have any impact on the number of people over the age of 65, so like answer choice (C), this answer has no effect on the argument.

Answer choice (E): Comparing the average age of the region to that of surrounding regions does not affect our opinion of the possibility that there are now more people over 65 in the region than there were 10 years ago. That is, just because the average age is higher than that of surrounding regions does not mean that the number of people over the age of 65 has increased.

Courtesy: PowerScore

EXPLANATION FROM Fox LSAT

This is traditionally a very difficult question for would-be lawyers, partially because it contains numbers. You must get over your math-phobia and argue with the logic here. It’s still a hard question, but you narrow it down to a couple answers.

The conclusion is the first sentence: “Over the last 10 years, there has been a dramatic increase in the number of people over the age of 65 living in this region.” The evidence is that during this time the average age has increased from 52 to 57.

Does this evidence prove this conclusion? Hell no. Just because the average age increased slightly doesn’t mean there are now way more retirees in town. There are plenty of other explanations. Here are a couple:

1) Let’s say, just for fun, that a school bus containing every single one of the town’s cute little children drove straight into a volcano, killing everyone with lava. What would happen to the average age in town? It would go up. In this scenario, there would be the exact same number of people in town over the age of 65 and far fewer young people, driving the average age up.

2) What if nobody was born or died in town for ten years? And if nobody moved in or out? After ten years the average age would have gone up by exactly ten years. To comply with the given facts, it would actually have been necessary that a few of the old folks died, so that the average age would have only gone up by 5 years instead of 10. So in this scenario there would be less people over the age of 65, but most people would just be 10 years older than they were at the beginning, therefore the average age would have gone up.

The question asks us to strengthen the argument. We’re looking for something like the opposite of what I’ve predicted above. “No children died during the 10 year period,” might be a good answer, because it would defend against my volcano fantasy. Okay, let’s see what we’ve got:

A) I like this answer because it defends against my volcano idea above.

B) This is a weakener. It matches my example about nobody being born.

C) This could only possibly weaken the argument. If the total number of people in town has decreased over the last 10 years, that makes it even harder for the conclusion to be possible. This is out, because we want a strengthener.

D) This is a mild strengthener, because if there are more people in town overall then there are likely to be more old folks as well. The problem I see with this answer is that it’s compatible with a bunch of 60-year-olds moving into town. If a bunch of 60-year-olds moved in, then the average age would go up, but there would not be more 65-plusers. I like A better than this answer, so this one is out.

E) Surrounding regions are absolutely irrelevant to the conclusion of the argument. This is the easiest answer to discard.

A is the answer. It’s not perfect, but I think it’s the best of a bad lot.

Hello from the GMAT Club VerbalBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.