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by PAB2706 » Thu May 14, 2009 4:57 am
if i take x as the correct answer and 200-x as wrong answer i get a dif answer..but if i take x as the wrong answer and 200-x as the correct answer i cud solve it ..i think i am making some silly mistake
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by DanaJ » Thu May 14, 2009 5:33 am
200-question test?! WTF? But never mind...

I'll try both methods and compare the answers.

I. x is number of correct questions, 200 - x is the number of wrong/unanswered questions.
You get that x + 1/5 (200 - x) = 100
x + 40 - x/5 = 100
4x/5 = 60
x = 75 - so you need at least 75 correctly answered questions.

II. x is the number of wrong/unanswered questions and 200 - x is the number of correctly answered questions.
You get that 1/5 * x + 200 - x = 100
200 - 100 = x - x/5
100 = 4x/5
x = 125 - you are allowed have at most 125 wrongly answered questions, so you need at least 200 - 125 = 75 correct questions.

Seems I'm getting the same answer. My guess is that getting 75 and 125 for x confused you...
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by Ian Stewart » Thu May 14, 2009 6:59 am
DanaJ wrote: You get that x + 1/5 (200 - x) = 100
I think you might have misinterpreted the question - it asks us to subtract one fifth the number of incorrect answers, so we should have a minus sign:

x - 1/5 (200 - x) > 100
(6x/5) - 40 > 100
(6x/5) > 140
x > 5*140/6
x > 116.66....

So x must be at least 117. This is a question where you could also backsolve if you were uncertain how to set up the algebra. Considering the answer choices, you'd almost certainly want to start with either 116 or 117, since they're so close together - that suggests that one of them should be the right answer, while the other is there to trap someone who rounds down instead of up.

Pab2706, where are these questions from? You've posted a few recently, and some are poorly written (the percent question, for example), and one clearly had a mathematical error, as Dana pointed out (the x^2 + y^2 = 1 question).

__________

And an aside which is completely irrelevant to the current GMAT: the scoring system described in this question is similar to that which was often used in US standardized paper tests (SATs, for example, or the old GMAT paper tests). Those tests usually included a 1/4 point penalty for incorrect answers (though no penalty for unanswered questions) so that random guessing gave test takers neither an advantage nor a disadvantage - with five answer choices per question, if you guess at five questions, on average you'll get four wrong and one right, and therefore 1 point for the right answer, and a 4*(1/4) = 1 point penalty for the four wrong answers, meaning that 5 random guesses would, on average, neither increase or decrease your score. Of course that's not at all how the computer-based GMAT works, but you'll see that scoring system if you practice from an old GMAT paper test, and that's where the scoring system in the above question comes from.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by DanaJ » Thu May 14, 2009 7:08 am
Thanks, Ian. You're absolutely right.
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by PAB2706 » Thu May 14, 2009 8:35 am
hi Ian yes the answer is 117...

i got some question sets from a friend ( they are mix of old paper based gmat and some other test prep organization )... i have a major time management problem in quant....i can solve all the quant problem tht are seen on the gmat ..but my major issue is time and hence i just end up with Q 45-47 in my mock tests..and i am in search of maximum practice questions to improve my time management...if u can suggest any better material online i would be really grateful.

I am aware that the language is quite lousy in some problems( interest prob) while some problems have wrong answer options( x^2+y^2=1) and i guess that is why i am finding problem while solving such questions. Sorry for the inconvenience.

cheers!!
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by KICKGMATASS123 » Thu May 14, 2009 6:12 pm
Ian Stewart wrote:
DanaJ wrote: You get that x + 1/5 (200 - x) = 100
I think you might have misinterpreted the question - it asks us to subtract one fifth the number of incorrect answers, so we should have a minus sign:

x - 1/5 (200 - x) > 100
(6x/5) - 40 > 100
(6x/5) > 140
x > 5*140/6
x > 116.66....

So x must be at least 117. This is a question where you could also backsolve if you were uncertain how to set up the algebra. Considering the answer choices, you'd almost certainly want to start with either 116 or 117, since they're so close together - that suggests that one of them should be the right answer, while the other is there to trap someone who rounds down instead of up.

Pab2706, where are these questions from? You've posted a few recently, and some are poorly written (the percent question, for example), and one clearly had a mathematical error, as Dana pointed out (the x^2 + y^2 = 1 question).

-------------------------------------------------------------------------------------
Ian,

How do you know if you should round up on this one?

Thanks so much in advance!
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