## Video 38 Of RC Module

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### Video 38 Of RC Module

by javzprobz » Sun Jul 14, 2013 6:39 am
Hi Brent!

I have a quick question regarding the video 38 of RC module. The video is a RC question about cyclists. I think I really understand the somehow quantitative logic that we need in order to solve the question (I'm talking about the part that you explain the scenarios for cyclists involved in accidents and victims). Even though that I picked a wrong answer choice while solving the question on my own, I absolutely understood the question after your explanation. However,I wonder how you got to "less than 80% of cyclists in accidents were not wearing helmet" assumption. Yes, I understand that since 80% of the victims were not wearing helmet, the lower percentage of cyclists who were not wearing helmets in accidents we'd have, the better our conclusion would sound. So the part that I understand is that if out of 10,000 cyclists involved in accidents last year 8000 did not wear helmets, since 800 who died did not wear helmet, we would have 800/8000 (10%). If out of 10,000 cyclists involved in accidents last year 2000 did not wear helmets, we would have 800/2000 (40%). So the higher this percentage, the more logical our conclusion. However, I feel like if the answer choice C had said less than 70 or 60% (which is OK with your assumption since they are less than 80%), I would have not picked it. I feel like that our assumption should have mentioned less than 50%, not 80% (so only if C had said 10,20,30,40, or less than 50% I would have picked it (I know that C says 20% and that's why it is the answer).

I don't know if my question is clear. I would appreciate it if you enlighten me. Thanks a lot in advance.

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by Brent@GMATPrepNow » Sun Jul 14, 2013 7:17 am
Good question, javzprobz.

First of all, this is a VERY difficult CR question.

I'd like to answer your question with an example that's analogous to the helmet example.

Let's say that a study looked at all murders in the U.S. in 2012 and found that 80% of the murderers were right-handed.
Can we conclude that being right-handed increases one's likelihood of becoming a murderer? The answer depends on the proportion of right-handed people in the general population.
If it were the case that 80% of all people are right-handed, then it makes perfect sense that 80% of the murderers were right-handed. Here, the proportion of right-handed murderers is the same as the proportion of right-handed people in the general public. So, there's no connection between right-handedness and becoming a murderer.
However, If it were the case that 50% of all people are right-handed, then we'd expect the proportion of right-handed murderers to be about 50%. So, if the 80% of the murderers were right-handed, then we might conclude that there's a connection between right-handedness and one's potential for murder.
So, if 50% of people in the general public are right-handed, then our conclusion is very convincing.
If 5% of people in the general public are right-handed, then our conclusion is even more convincing.
If 60% of people in the general public are right-handed, then our conclusion is less convincing.

So, as you suggest, answer choice C would have been much more compelling if the percent were really low (like 10%). However, we're not trying to find an answer choice that guarantees the conclusion. We're just looking for an answer choice that strengthens the conclusion.

Since answer choice C is the only option that strengthens the conclusion, it must be the correct answer.

I hope that helps.

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com

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by javzprobz » Sun Jul 21, 2013 8:08 am
Got it. Thank you so much.

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