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The purpose of this problem is to exploit a weakness used by [#permalink]
17 Nov 2005, 22:11

1

This post received KUDOS

The purpose of this problem is to exploit a weakness used by ETS: 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. Duttist, GSR, Titleist, Laxie, Gamjatang, Wilfred, Nakib, etc... NO PENCILS allowed! Please break out the big guns and explain your deductive thought process.

#50: 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?

Well, my probability skills is not one that i can boast of
Looking at this, the number 678,463 seems to be a prime number (atleast not divisible by (2, 3, 4, 6, 7, 8, 9, 11...).
Also, 6^k is even.
On the exam I will go for E) 100%

Well, my probability skills is not one that i can boast of Looking at this, the number 678,463 seems to be a prime number (atleast not divisible by (2, 3, 4, 6, 7, 8, 9, 11...). Also, 6^k is even. On the exam I will go for E) 100%

E is not a LUCKY TWIN (no corresponding 0% among the answer choices) so no "Yukay" for you GSR.

well, let's see: because the second group contains the first 10 non-negative integers ...there's the case of 0 to be picked ..and the probability of picking 0 from this group is 1/10
The number 6.... (hik,don't remember) can only be a multiple of Z in case Z=1 , that is to say K=0 ..since K is the product of the two stated groups...the prob of K be 0 is 1/10 or 10%
So to let the conditon satisfied , the prob blah blah is 100%-10%=90%

Well, my probability skills is not one that i can boast of Looking at this, the number 678,463 seems to be a prime number (atleast not divisible by (2, 3, 4, 6, 7, 8, 9, 11...). Also, 6^k is even. On the exam I will go for E) 100%

E is not a LUCKY TWIN (no corresponding 0% among the answer choices) so no "Yukay" for you GSR.

for example: two answer choices X% and Y% , if X% + Y% =100% , they're lucky twins and possibly correct answer so pay much attention to these pairs. Those answer choices which can't find another choice to form lucky twins can be eliminated. Thanks to Matt for nice trick

Re: The purpose of this problem is to exploit a weakness used by [#permalink]
27 Feb 2014, 13:21

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