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dineshyamato
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JimGMATAnswers
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This is great! Could you add more traps if you happened to notice!!

+Kudos! :)
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dineshyamato
Some generic Quant points

Wow, I wish found this post before my exam
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Nice post.
Thanks.
Kudos and a bump to the post.
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dineshyamato

7. The lucky twin (thanks to, and suggested by GMATT73)
The purpose of this problem is to exploit a weakness used by PVue: complimentary answer choices. Almost always in complimentary probability questions, there are a pair of "LUCKY TWINS" among the answer choices. If in doubt and pressed for time, choose a TWIN by logical deduction.

Let`s take a crack at this Project GMAT bad boy without making lengthy calculations.

Set S consists of numbers 2, 3, 6, 48, and 164. Number K is computed by multiplying one random number from set S by one of the first 10 non-negative integers, also selected at random. If Z=6^K, what is the probability that 678,463 is not a multiple of Z?

a. 10%
b. 25%
c. 50%
d. 90%
e. 100%

*LUCKY TWINS




Can someone please explain this? What does the author mean by 'Lucky Twins in complimentary probability questions'??
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rajsekhark
Can someone please explain this? What does the author mean by 'Lucky Twins in complimentary probability questions'??

There's a 'guessing strategy' I've seen recommended in some prep materials, sometimes called a "twins strategy", or, as I suppose it's called here, a "lucky twins" strategy. The theory is that if you're asked to find something like "the probability you pick a woman from a group of employees," and the answer is actually "1/3", that the test writers will include both "1/3" and "2/3" among the answer choices. That's allegedly done to "trap" test takers who solve for the probability of picking a man instead of a woman.

The problem with that "twins strategy" is it doesn't work. I tested the strategy empirically -- I took every question from one edition of the OG and QR books where the strategy might be used, and calculated how often you'd guess correctly using it. It turned out on that pool of questions you'd get a question right 19.75% of the time by guessing one of the "twins". That's slightly worse than the 20% you'd get right by guessing purely at random. The difference isn't statistically significant, but there's no evidence at all that this strategy is even slightly helpful; there are enough official questions where both "twins" are wrong that using this strategy does not improve your odds of answering questions correctly. So you can safely ignore anything about "twins".
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IanStewart
rajsekhark

There's a 'guessing strategy' I've seen recommended in some prep materials, sometimes called a "twins strategy", or, as I suppose it's called here, a "lucky twins" strategy. The theory is that if you're asked to find something like "the probability you pick a woman from a group of employees," and the answer is actually "1/3", that the test writers will include both "1/3" and "2/3" among the answer choices. That's allegedly done to "trap" test takers who solve for the probability of picking a man instead of a woman.

That was explained brilliantly well!
A special acknowledgement to you conducting the trials for all those questions...

Thank you so much!
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Thanks dineshyamato, I was able to guess the below using Lone wolf.
https://gmatclub.com/forum/if-q-s-and-t ... 42956.html
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This is a great list! super helpful. thanks to the poster.
Wondering if anyone else has a list/ book/ document that covers all potential traps in GMAT? if something like that exists.
I have heard in some courses the tutors reveal traps to you if you buy their course (I took their trial classes), so just shooting my shot here once.
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