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Probability Problem

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by neelgandham » Wed Nov 02, 2011 7:28 am
7/20 ?
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by shankar.ashwin » Wed Nov 02, 2011 7:38 am
Prime and odd - 3,5,7,11,13,17,19

7/20
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by satishchandra » Wed Nov 02, 2011 8:12 am
Prime and odd: 3,5,7,11,13,17,19

Probability = 7/20
odds in favor = no.of favorable cases/no.of unfavorable cases= 7/13
odd against = no.of unfavorable cases/no.of favorable cases= 13/7

odds of selecting prime and odd number ~ odds in favor = 7/13
Ans is 7/13
Whats the OA?
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by frumpychic » Wed Nov 02, 2011 8:41 am
what about "2"? That is prime but not odd. Do we add that back?
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by satishchandra » Wed Nov 02, 2011 8:52 am
frumpychic wrote:what about "2"? That is prime but not odd. Do we add that back?
NO. you should not add 2 in your list as it is not odd.

We have the question as prime AND odd (Pay special attention to 'AND' here)
Had it been Prime or odd, then we would have considered all primes and odd numbers in the list.

Whats OA btw?
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by studentps2011 » Wed Nov 02, 2011 12:33 pm
In Gmat questions, is there a differentiation between the terms 'odds' and 'probability' or are they both used to refer to probability?

I have seen probability questions where the term 'odds' is used. At the same time, Kaplan uses them to refer to only 'odds'

For example, in the above question,

odds of selecting prime and odd number = 7/13
whereas,
probability of selecting prime and odd number = 7/20

Experts, advise please!
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by GmatMathPro » Wed Nov 02, 2011 12:56 pm
studentps2011 wrote:In Gmat questions, is there a differentiation between the terms 'odds' and 'probability' or are they both used to refer to probability?

I have seen probability questions where the term 'odds' is used. At the same time, Kaplan uses them to refer to only 'odds'

For example, in the above question,

odds of selecting prime and odd number = 7/13
whereas,
probability of selecting prime and odd number = 7/20

Experts, advise please!
I would be extremely surprised if the GMAT ever had a question that tested whether you know the difference between odds and probability. In any case, though, probability is always a number between 0 and 1 that calculates the favorable outcomes divided by the total outcomes. Odds, on the other hand, is the ratio of favorable outcomes to unfavorable outcomes, or unfavorable outcomes to favorable outcomes. For example, if we pick an integer at random from the numbers 1,2,3,4,5. The probability of picking an odd number is 3/5. The odds in FAVOR of picking an odd number are 3:2. The odds AGAINST picking an odd number is 2:3.

Odds and probability are not synonyms, so a real GMAT question would never use the words interchangeably. Any practice question that does is sloppily written.
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by Brent@GMATPrepNow » Wed Nov 02, 2011 1:13 pm
studentps2011 wrote:In Gmat questions, is there a differentiation between the terms 'odds' and 'probability' or are they both used to refer to probability?

I have seen probability questions where the term 'odds' is used. At the same time, Kaplan uses them to refer to only 'odds'

For example, in the above question,

odds of selecting prime and odd number = 7/13
whereas,
probability of selecting prime and odd number = 7/20

Experts, advise please!
I agree with GmatMathPro.

I should also add that I don't believe I've ever seen an official GMAT question require us to determine "the odds"

Also, I just checked the OG12 and not one math question asks you to find "the odds."

Cheers,
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by studentps2011 » Wed Nov 02, 2011 1:20 pm
Thanks Brent & GmatMathPro!

I came across the below problem from Kaplan, which confused me. I could only solve it using probability and then I got the wrong answer!
For each 6-month period during a light bulb's life span, the odds of it not burning out from over-use are half what they were in the previous 6-month period. If the odds of a light bulb burning out during the first 6-month period following its purchase are 1/3, what are the odds of it burning out during the period from 6 months to 1 year following its purchase?

A. 5/27
B. 2/9
C. 1/3
D. 4/9
E. 2/3

OA:D
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