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Updated on: 08 May 2015, 07:29
Originally posted by bjshukla on 02 Jan 2011, 13:08.
Last edited by Bunuel on 08 May 2015, 07:29, edited 1 time in total.
Last edited by Bunuel on 08 May 2015, 07:29, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
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If x and y are prime numbers, is y(x-3) odd?
(1) x > 10
(2) y < 3
(1) x > 10
(2) y < 3
Hello,
Book: Kaplan GMAT Premier 2011
Pages: 457 and 458
For the data sufficiency problem "If x and y are prime numbers, is y(x-3) odd?" on page 457, explained answer on page 458 is D (Each statement alone is sufficient.)
2 statements are:
1. x > 10
2. y < 3
However, I feel that answer should be A (only 1 is sufficient).
Statement 2 is not sufficient, because:
1. If we pick y = 2 and x = 2, equation yields 2(-1) = -2 => Even number
2. If we pick y = 1 and x = 2, equation yields 1(-1) = -1 => Odd number
Thus depending on what we pick, the result can be even or odd...hence we cannot conclusively say if y(x-3) will be odd or not based on statement 2.
May you please advise if I am not approaching this correctly?
Thank you for your help in advance!
Book: Kaplan GMAT Premier 2011
Pages: 457 and 458
For the data sufficiency problem "If x and y are prime numbers, is y(x-3) odd?" on page 457, explained answer on page 458 is D (Each statement alone is sufficient.)
2 statements are:
1. x > 10
2. y < 3
However, I feel that answer should be A (only 1 is sufficient).
Statement 2 is not sufficient, because:
1. If we pick y = 2 and x = 2, equation yields 2(-1) = -2 => Even number
2. If we pick y = 1 and x = 2, equation yields 1(-1) = -1 => Odd number
Thus depending on what we pick, the result can be even or odd...hence we cannot conclusively say if y(x-3) will be odd or not based on statement 2.
May you please advise if I am not approaching this correctly?
Thank you for your help in advance!
Pure algebraic questions like this one are no longer part of the GMAT Data Sufficiency syllabus.
For relevant Data Sufficiency questions, please check the Data Sufficiency forum:
Data Sufficiency Forum
Data Sufficiency Forum
bjshukla
If x and y are prime numbers, is y(x-3) odd?
Note that we are told that both x and y are prime numbers, also note that 1 is not a prime number.
Now, in order the product of 2 integers to be odd both must be odd, so y(x-3) to be odd y must be any odd prime and x must be the only even prime 2, so that x-3=even-odd=odd. In all other cases given product will be even.
(1) x > 10 --> x is not 2, so the product is even. Sufficient.
Or: x is a prime number more than 10, so it's odd --> x-3=odd-odd=even --> y(x-3)=y*even=even.
(2) y < 3 --> the only prime less than 3 is 2, so y(x-3)=even*(x-3)=even. Sufficient.
Answer: D.
For more check Number Theory chapter of Math Book: math-number-theory-88376.html
Hope it helps.
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1. Only even prime is 2, so if x>10 then (x-3) is even. SUFFICIENT.
2. y<3 does not prove anything. NOT SUFFICIENT.
Hence ans is A
2. y<3 does not prove anything. NOT SUFFICIENT.
Hence ans is A
Bunuel
I have a question, Can we disregard -ve numbers here for part 2. (y<3) ?
reatsaint
Yes, because it is mentioned in the stem that "x and y are prime numbers". Prime numbers are never -ve;
Per GMATClub Math Book:
A Prime number is a natural number with exactly two distinct natural number divisors: 1 and itself.
Posts: 91
Updated on: 07 Jul 2013, 21:50
Originally posted by laserglare on 07 Jul 2013, 21:40.
Last edited by Bunuel on 07 Jul 2013, 21:50, edited 1 time in total.
Last edited by Bunuel on 07 Jul 2013, 21:50, edited 1 time in total.
Renamed the topic, edited the question and moved to DS forum.
If x and y are prime numbers, is y(x-3) odd?
(1) x > 10
(2) y < 3
This is from the Kaplan 2013 book, page 562 #11
(1) x > 10
(2) y < 3
This is from the Kaplan 2013 book, page 562 #11
The answer is D
I put B. I don't understand how (1) alone can be sufficient. Because when you plug in any prime number for x>10 using (1) guidelines, you will get an ODD number for (x-3) however we don't know the value of y, what if its 2 or 3, that can change the yes/no answer to the question. Please help the explanation is not thorough and its been bugging me for so long.
I put B. I don't understand how (1) alone can be sufficient. Because when you plug in any prime number for x>10 using (1) guidelines, you will get an ODD number for (x-3) however we don't know the value of y, what if its 2 or 3, that can change the yes/no answer to the question. Please help the explanation is not thorough and its been bugging me for so long.
laserglare
If x and y are prime numbers, is y(x-3) odd?
In order the product of 2 integers to be odd, both must be odd. So, y(x-3) to be odd, y must be any odd prime and x must be the only even prime 2, so that x-3=even-odd=odd. In all other cases given product will be even.
(1) x > 10 --> x is not 2, so the product is even. Sufficient.
Or: x is a prime number more than 10, so it's odd --> x-3=odd-odd=even --> y(x-3)=y*even=even.
(2) y < 3 --> the only prime less than 3 is 2, so y(x-3)=even*(x-3)=even. Sufficient.
Answer: D.
P.S. Please read carefully and follow: rules-for-posting-please-read-this-before-posting-133935.html Thank you.
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Posts: 91
Ok, i'll follow rules next time, srry new to forum.
anyways i understand now, feel dumb lol
anyways i understand now, feel dumb lol
Posts: 22
Hi Banuel,
why are we presuming one of x and y to be 2 ?
for statement 1 : x is odd, so x-3 will be odd. y can also be odd, why do we presume it to be 2 ?
pl help.
why are we presuming one of x and y to be 2 ?
for statement 1 : x is odd, so x-3 will be odd. y can also be odd, why do we presume it to be 2 ?
pl help.
vishalrastogi
We are not doing that.
y(x-3) to be odd, y must be any odd prime and x must be the only even prime 2, so that x-3=even-odd=odd.
Also, if x is odd, then x-3=odd-odd=even.
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Posts: 22
Thanx Bunuel. I suddenly feel stupid. odd-odd=even, how silly of me.
Are there no negative prime numbers? Because if there are negative prime numbers, the answer to this question would be A.
aakrity
No, only positive integers can be primes. The smallest prime (and the only even prime) is 2. For more check here: math-number-theory-88376.html
Hope it helps.
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jlgdr
Joined: 06 Sep 2013
Last visit: 24 Jul 2015
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Concentration: Finance
Schools: Wharton '17 (A) HBS '17 (WA$)
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laserglare
Take a look at the question is y(x-3) odd?
Now, x,y both primes right? So x-3 will always be even except when x = 2, only even prime
(1) x>10 so x is not 2
Suff
(2) y<3 so 'y' has to be 2 cause is the smallest prime number
Remember negatives can't be prime numbers
1 is of course not prime either
Hence D here
Cheers!
J
EMPOWERgmatRichC
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Expert reply
Hi All,
This is a great Number Property question; even if you don't immediately recognize the Number Properties involved, you can still discover the patterns (although it might take a little work).
We're told that X and Y are PRIME NUMBERS. We're asked if Y(X-3) is ODD. This is a YES/NO question.
Fact 1: X > 10
Since we know that X is PRIME, this Fact tells us that X must also be ODD. Y can be ANY PRIME number....
IF....
X = 11
then (X-3) = (11-3) = 8
(any prime)(8) will be EVEN, so the answer to the question is NO.
IF....
X = 13
then (X-3) = (13-3) = 10
(any prime)(10) will be EVEN, so the answer to the question is NO.
IF....
X = 17
then (X-3) = (17-3) = 14
(any prime)(14) will be EVEN, so the answer to the question is NO.
This pattern continues on; the answer to the question is ALWAYS NO.
Fact 1 is SUFFICIENT
Fact 2: Y < 3
Since Y is PRIME, we know that Y MUST be 2. X can be ANY PRIME number....
IF....
Y = 2 and X = ANY PRIME
then (X-3) = an integer
(2)(any integer) will be EVEN, so the answer to the question is ALWAYS NO.
Fact 2 is SUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich
This is a great Number Property question; even if you don't immediately recognize the Number Properties involved, you can still discover the patterns (although it might take a little work).
We're told that X and Y are PRIME NUMBERS. We're asked if Y(X-3) is ODD. This is a YES/NO question.
Fact 1: X > 10
Since we know that X is PRIME, this Fact tells us that X must also be ODD. Y can be ANY PRIME number....
IF....
X = 11
then (X-3) = (11-3) = 8
(any prime)(8) will be EVEN, so the answer to the question is NO.
IF....
X = 13
then (X-3) = (13-3) = 10
(any prime)(10) will be EVEN, so the answer to the question is NO.
IF....
X = 17
then (X-3) = (17-3) = 14
(any prime)(14) will be EVEN, so the answer to the question is NO.
This pattern continues on; the answer to the question is ALWAYS NO.
Fact 1 is SUFFICIENT
Fact 2: Y < 3
Since Y is PRIME, we know that Y MUST be 2. X can be ANY PRIME number....
IF....
Y = 2 and X = ANY PRIME
then (X-3) = an integer
(2)(any integer) will be EVEN, so the answer to the question is ALWAYS NO.
Fact 2 is SUFFICIENT
Final Answer:
GMAT assassins aren't born, they're made,
Rich
Tmoni26
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Hi Rich,
So is this question telling me that Prime numbers cannot be -ve numbers??
Is that also true for the GMAT
So is this question telling me that Prime numbers cannot be -ve numbers??
Is that also true for the GMAT
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Expert reply
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Tmoni26
That's true in all of math, GMAT or otherwise - prime numbers are never negative. The smallest prime is 2.
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Tmoni26
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Hi IanStewart,
Thanks a lot for that clarification....
Thanks a lot for that clarification....
Hi,
Maybe somebody can explain me, whi we can't take x=3 in the (2)? In this case it would be 2(3-3) = 0 => insufficient
Hence, the answer is A
Why it's not like that?
Maybe somebody can explain me, whi we can't take x=3 in the (2)? In this case it would be 2(3-3) = 0 => insufficient
Hence, the answer is A
Why it's not like that?
anna3565
ZERO.
1. 0 is an integer.
2. 0 is an even integer. An even number is an integer that is "evenly divisible" by 2, i.e., divisible by 2 without a remainder and as zero is evenly divisible by 2 then it must be even.
3. 0 is neither positive nor negative integer (the only one of this kind).
4. 0 is divisible by EVERY integer except 0 itself.
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Ultimate GMAT Quantitative Megathread
Hope it helps.
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