nhemdani wrote:

GMATT73 wrote:

You can add in "Lucky Twins" to your repertoire

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 TWINSHi there can you please expand on this point? I didnt quite get why 10 and 90 are complimentary. In general how do you figure twins?

10% and 90% are complements of each other. They together make 100%. If the answer to a question is 10%, there are chances that 90% is also an option. It's a trap since someone who calculates the complement might forget to subtract it from 100%.

e.g. Say, you need to find the probability that event A occurs. Instead, you find it easier to calculate the probability that event A does not occur (say you get this as 0.3). After that you need to subtract the probability that A does not occur from 1 to get the probability that A occurs i.e. you get 0.7 which will be your answer.

It is probable that the answer options will include 0.3 as well to trap you if you forget to subtract it from 1 at the end.

So if complements are present, there is a chance that once of those two will be the answer. Mind you, this is pure guess work. Use it only and only if you have 10 sec to take a guess and move on. Or if you need to plug in options, you can try these first.

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Karishma

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