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A and B, both have to select any integer from x to y, inclusive

The total number of integers is given by n = y-x+1 > we add 1 to account for the last digit since both x and y are included in this range

The total number of selections that can be made = (Total # of choices that A has)*(Total # of choices that B has) = \(n^2\)

The probability that A's choice is greater than B's choice is given by:
\(\frac{number of favorable outcomes}{n^2}\)

Let's look at the statements now:

Statement 1:
y−x=9

This gives us, n=y-x+1 = 9+1 = 10

Now, that we have 10 (meaning their are 10 integers for both A and B to select from), let's calculate the probability:

Assume, x=0 and y=10

  • If B picks 10, 0 favorabe outcome (since, A cannot pick an integer greater than 10)
  • If B picks 9, 1 favorable outcome (since A can pick 10)
  • If B picks 8, 2 favorable outcomes (since A can pick any of 9 or 10)
  • And so on..

Adding up all the favorable outcomes we get = 0+1+2+....9
This is nothing but the sum of N natural numbers given by \(\frac{N*(N+1)}{2}\)

Since N=9, Sum=\(\frac{9*10}{2}\)=45

The probability is now: \(\frac{number of favorable outcomes}{n^2}\) = \(\frac{45}{100}\) = 0.45

Sufficient.

Statement 2:
y=−20

This statement tells us nothing about x, and consequently, nothing about the total number of integers we have to choose from. Hence, insufficient.

Correct answer is A.
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Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20



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Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

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We need to find the probability that Ann selecting is greater than the one Bob.

Stmt 1 : y - x = 9
So we have a set of 10 numbers. It can be any number 1,2,3....10.

So, for the favorable probability, we need to have a 10C2, ignoring the order.

Hence, statement 1 is self-sufficient.

Stmt 2: y = -20
As we don't have the range, we don't know the total number of cases to count for.
Hence, statement 2 is not sufficient.

IMO A.
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Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

Win over $30,000 in prizes such as Courses, Tests, Private Tutoring, and more

 

  • Statement (1) gives
    [ltr]
    yx=9[/ltr]
    .
  • The number of integers is
    [ltr]
    N=yx+1=9+1=10[/ltr]
    .
  • The total number of outcomes is
    [ltr]
    N^2=10^2=100 [/ltr]
    .
  • The number of outcomes where Ann's integer is greater than Bob's is
    [ltr]
    [N(N−1)]/2=[10(10−1)]/2=(10×9)/2=45[/ltr]
    .
  • The probability is
    [ltr]
    45/100=9/20[/ltr]
    .
  • Statement (1) is sufficient.

Analyze Statement (2) alone.

  • Statement (2) gives
    [ltr]
    2y=-20[/ltr]
    , so
    [ltr]
    y=-10[/ltr]
    .
  • This statement does not provide information about
    [ltr]
    x[/ltr]
    .
  • We cannot determine the number of integers in the range.
  • Statement (2) is not sufficient.
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Statement (1): y−x=9
So there are 9+1=10 total integers.
This gives us the total number of possible outcomes and allows us to count how many outcomes satisfy A>B.
Because the set is all integers are equally likely, this is enough to calculate the probability.
Statement (1) alone is sufficient.

Statement (2): y = −20
This tells us nothing about x.
So the number of integers is unknown.
Without knowing this, we cannot determine the probability.
Statement (2) alone is not sufficient.

Answer A
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Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

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asked: Probability of Ann Selecting a number greater than Bob.

Statement 1:
Range of number = 9

But there is no mention of repeating integers like 2,2,2,3,4,11
Hence B,C,E

Statement 2:
y = -20
No mention of any range of integers.
Eliminate B

Combined 1 and 2:
y= -20
y-x = 9 => x = - 29
Again no mention of any repeating integers Hence Correct option E.
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1. if y=x+9, it shows that there are 11 possible values for any given x or y. Therefore the probability can be calculated.

2. y=-20, this gives a infinite range of possible values which Ann and Bo can select, hence probability cannot be determined.

An Insight - Had the word 'integer' not mentioned in the question stem, the answer would be E, as for option 1 we will again get infinite number of possibilities.
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Visualize the numbers on a number line from x to y, both inclusive

<--|--|--|--|-.......-|--|--|--|--|-->
‎ ‎ ‎ ‎ ‎x‎ ‎ ‎ ‎‎ ‎‎ ‎‎ ‎ ‎ ‎z ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎‎ ‎‎ ‎‎ ‎‎ ‎‎ ‎ ‎ ‎ ‎‎ ‎‎‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎ ‎y

The total number of possible integers that Ann or Bob can select is y-x+1 (which is total number of integers between 2 numbers, both inclusive)

Suppose Ann selects a specific number, z. Then, the number of integers Bob could select that are less than Ann’s choice is z-x [all the numbers to the left of z].

So, the probability that Bob picks a number less than Ann’s choice when Ann picks z is:
\(\frac{z-x}{y-x+1}\)

Thus the total probability of such selection can be figured out by adding the probability of each of that particular case, for which one would only need the total number of integers between x and y, both inclusive.

Therefore, only statement 1 is required (A).
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Answer: Statement (1) alone is sufficient → choice A.
[hr]
If there are n consecutive integers to choose from, the probability that Ann’s pick is greater than Bob’s is
P(Ann>Bob) = (n−1)/2n.
(Reason: of the n^2 ordered pairs (A,B), exactly n(n−1)/2 have A>B; see symmetry argument below.)
So we really just need the count of available integers, n=y−x+1
[hr]
Evaluate the statements
  1. y−x=9
    Then n=(y−x)+1=9+1=10
    P(Ann>Bob) = (10−1)/(2*10)=9/20
    The probability is fully determined. Statement (1) is sufficient.
  2. y=−20
    Without knowing x we don’t know n, so we can’t compute the probability.
    Statement (2) is not sufficient.
Since statement (1) alone suffices and statement (2) does not, the correct GMAT data-sufficiency answer is A.

Symmetry check for the formula: by symmetry P(A>B)=P(B>A).
Total probability 1=P(A>B)+P(B>A)+P(A=B)
Because P(A=B)=1/n, we get 2P(A>B)=1−1/n ⇒ P(A>B)=(n−1)/2n
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The probability that Ann selects a greater integer than Bob depends on the number of integers in the range from x to y inclusive denoted as k= y-x+1.
The probability is given by the formula (k-1)/2k
Statement 1: y-x=9
since k=y-x+1 by substitution we get probability 9/20
Thus, statement (1) alone is sufficient to determine the probability.

Statement 2: y=-20
This gives the value of
y, but x is unknown. The number of integers k=y-x+1 depends on x, and since
x can vary, k can take any integer value greater than or equal to 1.
Since different values of x yield different probabilities, statement (2) alone is insufficient to determine the probability.
Statement (1) alone is sufficient, but statement (2) alone is not sufficient.
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Condition 1: y - x = 9
This indicates that there are infinitely many combinations of y and x. Without knowing the range, it is impossible to deduce the possible combinations of Ann and Bob, and thus, the probability of one being greater than the other cannot be determined.

Condition 2: y = -20
Since we do not know x, the condition is insufficient.

Condition 1 + 2: Now we know y = -20 and x = -29, so the total number of combinations is 10*10.
Additionally, listing them out:
When Ann is -20, Bob has combinations from -21 to -29.
When Ann is -21, Bob has combinations from -22 to -29.
And so on. The conditions are now sufficient.
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D) You need both.

These equations let you determine the range of selection, and the probability is 0,5 (random) - chance of getting the same number (which is (1/range)^2).
Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

Win over $30,000 in prizes such as Courses, Tests, Private Tutoring, and more

 

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total no. of integers = y-x+1

prob both select the same = 1*(1/ y-x+1)

so, they choose different nos. = 1- (1/(y-x+1))

now, each have 50% chance= 0.5*( 1- (1/(y-x+1)))

1. y-x given.. reqd prob = 0.5*0.9...Sufficient
2. y=-20... not sufficient

Answer A
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1) y-x = 9

total 10 integers
no ways Ann & Bob can select integers = 10*10 = 100

no of ways integer selected by Ann grater than Bob = 1+2+...+9 = 45

P = 0.45

sufficient

2) y = -20

insufficient

Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

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statement 1 , we can take any range of difference 9 as - (0, 9), (10,19), (20,29) etc..so probability would be the SAME. sufficient
statement 2- we don't have any idea about x so insufficient.
if I am incorrect, please correct:|
Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

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C. both statements are needed to solve as substitute Y=-20 in one and then solve


Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

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Bunuel
Ann and Bob each randomly select an integer from the integers x to y, inclusive. What is the probability that the integer Ann selects is greater than the one Bob selects?

(1) y - x = 9
(2) y = -20


 


This question was provided by GMAT Club
for the GMAT Club Olympics Competition

Win over $30,000 in prizes such as Courses, Tests, Private Tutoring, and more

 

to know the probability we only need to know the total of numbers that are available to select. 1 gives us that information
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