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Re: Trouble with Quant? Some tips/traps that might help you... [#permalink]

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03 Oct 2010, 01:03

Rubyswtgrl wrote:

Is the answer for Lucky Twin question D?

I first thought it was E, thinking that 6^anything would be an even number, hence 678,463 would not be a multiple of an even number.

But I googled the answer and since the question lists set S as the first 10 non-negative integers, set S *includes* 0 (sneaky question, I was thinking 1, 2, 3, etc.). So since 0 has a 10% chance of being selected, K could = 0, making Z equal to 1, and 678,463 is a multiple of 1. So D = 90% is the correct answer. I guess Lucky Twin worked here , but my first choice still would have been the above.
_________________

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

I don't understand what you mean by "lucky twins". How does that work?

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

I don't understand what you mean by "lucky twins". How does that work?

Detailed answer: Total universe of possibilities for K: [a # from set S]*[0,1,2,..9] = [2*0 = 0, 2*1 = 2, .. 2*9=.... 3*0= 0, 3*1= 3...168*0, 168*1... 168*9] = total of 50 possible values of k. Out of that we'd have 5 0's. Probability of k being 0 = 5/50 = 1/10. Also, as mentioned in one of the previous posts, 6^0 =1 and 678 or 463 are multiples of 1. So there is a 10% chance that the numbers 678, 463 will be a factor of 6^k. So ans = 100-10 = 90 or d.

Now using the 'logic' of Lucky Twins - 10 and 90 are the only options out of the 5 that are complementary (10=100-90). None of the other options have their complements as options. So by the theory of Lucky Twins we can deduce that the answer is most probably either a or d. Also, since the units digit of 6^k will always be 6 as long as k != 0, the probability of 6^k NOT being a multiple of 678 or 463 should be pretty high. So the answer must be d or 90% and not 10%.

Re: Trouble with Quant? Some tips/traps that might help you... [#permalink]

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08 Apr 2011, 01:09

1

This post received KUDOS

First, let me congratulate necromonger for the enormous amount of detail shared. I think the original post is simply outstanding.

I'd also want to add two specific traps that I've frequently encountered:

1. In questions where Diameter and radius are involved, watch out carefully for what exactly the answer and the question deal in and ensure that the translation of the values between radius and diameter are done at every stage of the solution.

2. One often assumes integer values even if the question is not explicit about the nature of the numbers involved. I did this and have got some practise questions wrong

3. Venn-diagram questions and Algebra questions often appear very similar.

I will add more later to this list.
_________________

Can somebody please explain me the lone wolf problem with the solution.

Here you go:

On a loan, evil necromonger charges X% interest in the first year, and Y% interest in the second. If he loaned Rhyme 20,000$ in 2006, how much Rhyme pay by interest in 2008?"

A) X = 10 B) (X + Y + XY/100) = 100

The intent of the question was to say the following: On a deposit of $20,000, interest is paid at a rate of X% in the first year and at Y% in the second year. What is the total interest paid by the end of second year if no money was withdrawn?

On 20,000, you pay X% interest in the first year. Interest paid in first year = (X/100) * 20,000

Next year, you pay Y% interest on the total amount. Interest paid in the second year = (Y/100) * [20,000 + (X/100) * 20,000] = (Y/100)*20,000 + (XY/10,000)*20,000

Total interest paid in the two years = (X/100) * 20,000 + (Y/100)*20,000 + (XY/10,000)*20,000 = 200 * (X + Y + XY/100)

All you need to answer the question is the value of '(X + Y + XY/100)' which statement 2 gives you.
_________________

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