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Re: DS: probability [#permalink]
18 Aug 2008, 13:26

1

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I think the answer is E.

First the inequality simplifies to x <= 20. 1/4x - 5 <=0 1/4x <= 5 4*(1/4x) <= 4*5 x <= 20

We need to know what the options are in set T before we know the probability of selecting a value for x that is <= 20.

vksunder wrote:

If x is to be selected at random from T, what is the probability that 1/4x - 5 <=0?

1. T is a set of 8 integers This doesn't give us enough information. We know the total number, but all numbers in Set T could be greater than 20 thus giving us probability of 0, or they could all be less than 20 giving us a probability of 1, or anything in between. INSUFFICIENT 2. T is contained in the set of integers from 1 to 25, inclusive INSUFFICIENT. We need to know how many of the integers in the set of integers from 1 to 25 make up T. since we do not know this, we do not know the total number of integers in the pool, and since some integers are higher than 20, some will satisfy the inequality and others will not. Once we know how many integers we would need to know which ones make up T.

TOGETHER: Insufficient. Together we know that T consists of 8 integers from 1 to 25, but because we could have all that are <= 20 and probability would be 1, or we could include any number of integers from 21 to 25 inclusive thus giving us many different probability values.

_________________

------------------------------------ J Allen Morris **I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.

Re: DS: probability [#permalink]
20 Aug 2010, 05:34

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Expert's post

fatihaysu wrote:

x2suresh wrote:

vksunder wrote:

If x is to be selected at random from T, what is the probability that 1/4x - 5 <=0?

1. T is a set of 8 integers 2. T is contained in the set of integers from 1 to 25, inclusive

1/4x - 5 <=0 x<=20 1) T is a set of 8 integers Can be any 8 integers >20 or can be any integers <20..

insuffcient. 2) {1,2,...25}

probablity = 20/25

B is the answer.

I defininetly agree with you however OA is E,

it is written from 1 to 25 so it means t= (1,2,3,4...23,24,25) why it is not B couldnt understand...

Bunuel Can you check and help us plz?

First of all: 1/4x - 5 <=0 should be written as 1/4*x - 5 <=0.

1. If x is to be selected at random from T, what is the probability that \frac{1}{4}*x-5\leq{0}?

\frac{1}{4}*x-5\leq{0} --> is x\leq{20}?

(1) T is a set of 8 integers. Clearly insufficient. (2) T is contained in the set of integers from 1 to 25, inclusive. Though the wording is a little bit strange but it means that set T is a subset of a set of integers from 1 to 25, inclusive. Set T can be {1,5,7} or {21,22,25}... Also insufficient.

(1)+(2) T can be set of 8 integers, which are ALL less than or equal to 20 and in this case P(x\leq{20})=1 or T can be set of 8 integers which are NOT ALL all less than or equal to 20 and in this case P(x\leq{20})<1. Not sufficient.

Answer: E.

You should spotted that there was something wrong with your approach as (1) say that T is a set of 8 integers and if (2) says that T is a set of integers from 1 to 25 inclusive, so set of 25 integers (as you suggested) then it would mean that statements contradict each other and on GMAT two statements never contradict.

Re: DS: probability [#permalink]
20 Aug 2010, 20:39

This is why I like the contradiction, one statement can hint at the parameters or bounds. If I had looked at statement 2 first, I probably would have forgotten about the limit that the number of the set is not yet defined. But you look at the other statement, and see they limit the set to 8 digits, and it sets off a lightbulb in my head. If the set had 1 number or 25 numbers, would give different answers.
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Re: DS: probability [#permalink]
06 Jan 2011, 10:11

If x is to be selected at random from T, what is the probability that 1/4x - 5 <=0? x<= 20

1. T is a set of 8 integers 2. T is contained in the set of integers from 1 to 25, inclusive

St.1 [IS] set of the same integers? integers less than 20? negative integers? St.2 [IS] 1-25 inclusive maybe all the integers are the same no. 25 or no. 10

With both, you can conclude that there is a set of 8 integers between the no. 1-25 inclusive. They can still be all the same integers or all int less than 20 or all int greater than 20 or consist of random proportion of int greater/less than 20. Therefore you can not conclude a definite answer so ---> (E)

Re: DS: probability [#permalink]
15 Jul 2013, 04:46

Bunuel wrote:

f x is to be selected at random from T, what is the probability that 1/4x - 5 <=0?

First of all: 1/4x - 5 <=0 should be written as 1/4*x - 5 <=0.

1. If x is to be selected at random from T, what is the probability that \frac{1}{4}*x-5\leq{0}?

\frac{1}{4}*x-5\leq{0} --> is x\leq{20}?

(1) T is a set of 8 integers. Clearly insufficient. (2) T is contained in the set of integers from 1 to 25, inclusive. Though the wording is a little bit strange but it means that set T is a subset of a set of integers from 1 to 25, inclusive. Set T can be {1,5,7} or {21,22,25}... Also insufficient.

(1)+(2) T can be set of 8 integers, which are ALL less than or equal to 20 and in this case P(x\leq{20})=1 or T can be set of 8 integers which are NOT ALL all less than or equal to 20 and in this case P(x\leq{20})<1. Not sufficient.

Answer: E.

You should spotted that there was something wrong with your approach as (1) say that T is a set of 8 integers and if (2) says that T is a set of integers from 1 to 25 inclusive, so set of 25 integers (as you suggested) then it would mean that statements contradict each other and on GMAT two statements never contradict.

Hope it's clear.

Bunuel,

Is it not possible to arrive at the answer by combining the two statements? Theoretically, we can identify all the subsets of 8 integers from 1 to 25, find the probability of X>=20 in all those subsets and add those probabilities.

Re: DS: probability [#permalink]
15 Jul 2013, 04:55

Expert's post

keenys wrote:

Bunuel wrote:

f x is to be selected at random from T, what is the probability that 1/4x - 5 <=0?

First of all: 1/4x - 5 <=0 should be written as 1/4*x - 5 <=0.

1. If x is to be selected at random from T, what is the probability that \frac{1}{4}*x-5\leq{0}?

\frac{1}{4}*x-5\leq{0} --> is x\leq{20}?

(1) T is a set of 8 integers. Clearly insufficient. (2) T is contained in the set of integers from 1 to 25, inclusive. Though the wording is a little bit strange but it means that set T is a subset of a set of integers from 1 to 25, inclusive. Set T can be {1,5,7} or {21,22,25}... Also insufficient.

(1)+(2) T can be set of 8 integers, which are ALL less than or equal to 20 and in this case P(x\leq{20})=1 or T can be set of 8 integers which are NOT ALL all less than or equal to 20 and in this case P(x\leq{20})<1. Not sufficient.

Answer: E.

You should spotted that there was something wrong with your approach as (1) say that T is a set of 8 integers and if (2) says that T is a set of integers from 1 to 25 inclusive, so set of 25 integers (as you suggested) then it would mean that statements contradict each other and on GMAT two statements never contradict.

Hope it's clear.

Bunuel,

Is it not possible to arrive at the answer by combining the two statements? Theoretically, we can identify all the subsets of 8 integers from 1 to 25, find the probability of X>=20 in all those subsets and add those probabilities.

What is wrong in my approach here?

Thank you.

When a DS question asks to find some value, then the statement is sufficient ONLY if you can get the single numerical value.

Since we cannot get the single numerical value of the probability alsed, thee the answer is E.