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Current Student
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X and Y are prime integers. What is X + Y ? 1. X - Y is a prime integer 2. Y \lt X \lt 6 Source: GMAT Club Tests - hardest GMAT questions Is 1 considered a prime number? I selected C but was wrong. According to the answer: S2 is not sufficient. Consider X = 5 , Y = 3 and X = 3 , Y = 2 which is true... edit: but if you take both together... the only possible answer is X=5 and Y=3 and not X=3 and Y=2. The only way S2 and S1 is not sufficient is if we consider 1 a prime number... am I missing something?
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Current Student
Joined: 26 Jun 2006
Posts: 125
Schools: Ross Class of 2012
WE 1: Consumer Goods / IT
WE 2: Retail / Logistics
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Nevermind... I think I figured it out... I didn't consider 5-2 = 3
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Senior Manager
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sharmar wrote: X and Y are prime integers. What is X + Y ?
1. X - Y is a prime integer
2. Y \lt X \lt 6
(C) 2008 GMAT Club - m25#29
* Statement (1) ALONE is sufficient, but Statement (2) ALONE is not sufficient
* Statement (2) ALONE is sufficient, but Statement (1) ALONE is not sufficient
* BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient
* EACH statement ALONE is sufficient
* Statements (1) and (2) TOGETHER are NOT sufficient
Is 1 considered a prime number? I selected C but was wrong. According to the answer:
S2 is not sufficient. Consider X = 5 , Y = 3 and X = 3 , Y = 2 which is true...
edit: but if you take both together... the only possible answer is X=5 and Y=3 and not X=3 and Y=2. The only way S2 and S1 is not sufficient is if we consider 1 a prime number... am I missing something? 1) X - Y is prime case 1: 5 - 2 = 3 5 + 2 = 7 case 2: 7 - 5 = 2 7 + 5 = 13 Two answers. Insufficient. 2) Y < X < 6 Possible value for Y = {2, 3} Possible value for X = {3, 5} More than one answer. Insufficient. 1) & 2) Case 1: 2 < 3 < 6 3 - 2 = 1 ----- Not Prime Drop this case Case 2: 2 < 5 < 6 5 - 2 = 3 ----- Prime 5 + 2 = 7 Case 3: 3 < 5 < 6 5 - 3 = 2 ----- Prime 5 + 3 = 8 Two answers, Insufficient. The answer is E.
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SVP
Joined: 05 Jul 2006
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[quote="sharmar"]X and Y are prime integers. What is X + Y ?
1. X - Y is a prime integer
2. Y \lt X \lt 6
the difference between consecutive primes = /2/ except 2,3 = /1/
from 1
x,y could be any prime >3....insuff
from 2
obviously not suff
both
no idea if they are consec or not ...insuff
E is my answer
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CEO
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sharmar wrote: Nevermind... I think I figured it out... I didn't consider 5-2 = 3 ALSO CONSIDER 7-2 = 5 OR 7-5 = 2 SOME MORE 13 - 2 = 11 AND 13 - 2 = 11
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Senior Manager
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isnt 1 a prime number ?
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Intern
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No, 1 is not considered a prime. A prime number (or a prime) is a natural number that has exactly two distinct natural number divisors: 1 and itself. Posted from my mobile device 
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X and Y are prime integers. What is X+Y ?
1. X-Y is a prime integer
2. Y<X<6
Case 1: X-Y is prime number
Set (X,Y) : (5,2), (5,3),(7,2),(7,5),(13,2)(13,11).
No Unique Soln.
Case 2. Y<X<6
Set (X,Y) : (5,3),(5,2)(3,2)
No Unique Soln.
Combining Case 1 & Case 2 : no Unique Soln.
Thus E: Stmt 1 & Stmt 2 Together are not sufficient.
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Senior Manager
Joined: 17 May 2010
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Aaaah got it. Was down to C and E. Only considered the 5-3 = 2 solution, not 5-2 = 3 solution. Hence got C.
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Manager
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1 simple question : are negative prime numbers considered in GMAT ? If yes, this question can be solved in 30 sec....else the 3,2 explanation is correct
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Director
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Prime number is an integer number P >=2, divisible by only itself and 1.
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Some folks here say 1 is not a prime. But it is.
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sharmar wrote: X and Y are prime integers. What is X + Y ? 1. X - Y is a prime integer 2. Y \lt X \lt 6 Source: GMAT Club Tests - hardest GMAT questions Is 1 considered a prime number? I selected C but was wrong. According to the answer: S2 is not sufficient. Consider X = 5 , Y = 3 and X = 3 , Y = 2 which is true... edit: but if you take both together... the only possible answer is X=5 and Y=3 and not X=3 and Y=2. The only way S2 and S1 is not sufficient is if we consider 1 a prime number... am I missing something? If x and y are prime numbers, what is the value of x+y ? (1) x-y is a prime number --> if x=5 and y=2 (notice that in this case x-y=3=prime) then x+y=7 but if x=5 and y=3 (notice that in this case x-y=2=prime) then x+y=8. Not sufficient. (2) y<x<6 --> the same example as above is valid for this statement also. Not sufficient. (1)+(2) Again, the example from statement (1) is still valid and gives two different values for x+y. Not sufficient. Answer: E.
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Manager
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E it is....... Prime no roblem are always tricky!!!!! 
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I will go with E
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I would go with E
1 is definitely not a prime number
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Question can be answered by using both the statements together x=5 y=3 x-y=2 which is a prime number and yes 1 is not prime.. 
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Good question. I am particularly happy that I did end up solving this one is about a min and half. Followed the plugging in of values approach with values as 5,2,3 for statement (ii) and for statement (i) there are a whole number of possible options. Both statements together as well fail to give a concrete answer for the values 2,3,5. Hence chose E.
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Picking up on bunnels statement re: 1 being a prime.... If you have trouble remembering in the heat of battle, think: a prime number has 2 factors, 1 and itself. Although 1 has factors of 1 and iteslf, these are one and the same and therefore it doesn't have 2 (intiger) factors.
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