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TheUltimateWinner
Joined: 23 Feb 2015
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Schools: Harvard '24 Judge '23 LBS '24 McCombs '24 Mendoza Tepper '24 Ross '24 Cox Marshall '24 Fisher Kelley '24 Jones McDonough '24 Owen '24 Terry BU Simon '24 Katz GWU Mason Babson Gabelli Foster '24 Goizueta '24 Scheller Stern '23 CBS '23 Booth '23 Haas '25 Wharton '25 Stanford '26 Johnson'23 Fuqua '23 Kellogg '23 Kenan-Flagler'23 INSEAD Aug 22 intake Olin '23 Said '23 Sloan '23 Carey Arizona "22 Yale '23 UC Davis Madison '23 Anderson '23 Carroll Carlson '23 Darden '23
Posts: 2,464
06 Jul 2019, 09:02
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A
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
25%
(medium)
Question Stats:
69% (01:04) correct
31%
(01:31)
wrong
based on 61
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Are \(a\) and \(b\) co-prime?
1) \(LCM (a,b)=ab\)
2) \(a-b\) is prime
1) \(LCM (a,b)=ab\)
2) \(a-b\) is prime
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
06 Jul 2019, 09:20
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Are a and b co-prime?
Meaning does a and b have a GCF of 1?
(1)LCM(a,b) =a•b
Say a=5,b= 3—> LCM(2,3)= 5•3
but a=2, b= 4 —> LCM(2,4) is not 2•4
.: a and b will always be co-prime (sufficient )
(2) a-b = prime
Say a=5 , b=3—> 5-3= 2(prime)
a and b co-prime
Again, a=6 ,b=4 —> 6-4=2(prime)
a and b not co-prime
(Not sufficient)
Answer A
Posted from my mobile device
Meaning does a and b have a GCF of 1?
(1)LCM(a,b) =a•b
Say a=5,b= 3—> LCM(2,3)= 5•3
but a=2, b= 4 —> LCM(2,4) is not 2•4
.: a and b will always be co-prime (sufficient )
(2) a-b = prime
Say a=5 , b=3—> 5-3= 2(prime)
a and b co-prime
Again, a=6 ,b=4 —> 6-4=2(prime)
a and b not co-prime
(Not sufficient)
Answer A
Posted from my mobile device
07 Jul 2019, 05:00
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You don't need to know what "co-prime" means on the GMAT, but two numbers are "co-prime" if their GCD is 1. Since
GCD(a, b) * LCM(a, b) = ab
is always true, then if LCM(a, b) = ab, the GCD must be 1, and Statement 1 is sufficient.
Statement 2 is not sufficient, since two consecutive even integers (4 and 6 say) differ by 2, a prime, but do not have a GCD of 1, while two consecutive odd integers (5 and 7 say) differ by 2, a prime, and do have a GCD of 1.
GCD(a, b) * LCM(a, b) = ab
is always true, then if LCM(a, b) = ab, the GCD must be 1, and Statement 1 is sufficient.
Statement 2 is not sufficient, since two consecutive even integers (4 and 6 say) differ by 2, a prime, but do not have a GCD of 1, while two consecutive odd integers (5 and 7 say) differ by 2, a prime, and do have a GCD of 1.
