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14 May 2013, 03:30
Kudos
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E
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
75%
(hard)
Question Stats:
36% (01:51) correct
64%
(02:02)
wrong
based on 44
sessions
History
Date
Time
Result
Not Attempted Yet
If\(n_{C}_{r}\) gives a prime value,is \(r=n-1\)?
I. n is prime
II. r is not prime
Question created by me,constructive feedback welcome.Kudos to the first correct answer!
I. n is prime
II. r is not prime
Question created by me,constructive feedback welcome.Kudos to the first correct answer!
14 May 2013, 03:35
Kudos
Bookmarks
If\(n_{C}_{r}\) gives a prime value,is \(r=n-1\)?
I. n is prime
Case 3C1=3 \(r\neq{}n-1\)
Case 3C2=3 \(r=n-1\)
Not sufficient
II. r is not prime
Case 7C6=7 \(r=n-1\)
Case 7C1=7 \(r\neq{}n-1\)
Not sufficient
I+II) n is prime and r is not prime
Case 3C1=3 \(r\neq{}n-1\)
Case 2C1=2 \(r=n-1\)
Still not sufficient
E
I. n is prime
Case 3C1=3 \(r\neq{}n-1\)
Case 3C2=3 \(r=n-1\)
Not sufficient
II. r is not prime
Case 7C6=7 \(r=n-1\)
Case 7C1=7 \(r\neq{}n-1\)
Not sufficient
I+II) n is prime and r is not prime
Case 3C1=3 \(r\neq{}n-1\)
Case 2C1=2 \(r=n-1\)
Still not sufficient
E
14 May 2013, 11:17
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vinaymimani
Dear vinaymimani
Zarrolou already gave a brilliant solution. I'll just add --- this question doesn't strike me as particularly GMAT-like. Yes, it's a hard question, but I don't know --- it seems very abstractly mathematical and from what I can tell, the most straightforward solutions are simply plug-in of various single digit numbers, as Zarrolou did. On the most challenging math problems, it strikes me that the GMAT creates more elegant problems. It's easier to write a hard problem, but it's more difficult to write an elegant problem. That's just my 2¢
Mike
