[please note that =< stands for "less than or equal to"]
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If z is an integer, is z prime?
(1) 15! < z
(2) 17! + 2 =< z =< 17!+17
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i read the solution on the book, it might be because it's night and i'm tired now, but i didn't understand the explanation at all.....i suck at number properties O_o
could you tell me how long does it take you to figure out the right answer, and how do you do it?????i'm lost!
hard question from kaplan gmat DS chapter...
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- davedecibel
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- davedecibel
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ok after i read the explanation more carefully i got it... still, it's not really intuitive. maybe some of you can find out a better way? i'm curious....
Whats the ans ? Is it B?
17! + 2 = 17*16*....3*2*1 + 2 = 2(Z+1) =>divisible by 2... Hence not prime...
Similarly every other number can be grouped...
Except if the question was 17!+1... In that case I don't know how to proceed.
17! + 2 = 17*16*....3*2*1 + 2 = 2(Z+1) =>divisible by 2... Hence not prime...
Similarly every other number can be grouped...
Except if the question was 17!+1... In that case I don't know how to proceed.
- goyalsau
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TOPGMAT wrote:Whats the ans ? Is it B?
17! + 2 = 17*16*....3*2*1 + 2 = 2(Z+1) =>divisible by 2... Hence not prime...
Similarly every other number can be grouped...
Except if the question was 17!+1... In that case I don't know how to proceed.
Even i suppose to write this
17! + 1 , will not be divisible by any number from 2 to 17,
I think answer of this question should be B,
and please Daved , please share the Official explanation,
I am sure that will help a lot, till we don't have any other source of information..
Saurabh Goyal
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- selango
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stmt1,
15!<Z
No info about Z
Insufficient
stmt2,
17! + 2 <= z <= 17! + 17
17! = 1*2*3*...17
17!+2 = 2(1+1*3*..........17)
17! +2 is divisible by 2 .. not prime
Similarly check for 3,4,5....17
17! +3 is divisible by 3.. Not prime
17! +4 is divisible by 4..not prime
.......
....
17! +17 is divisible by 17.. not prime
Sufficient.
Pick B
15!<Z
No info about Z
Insufficient
stmt2,
17! + 2 <= z <= 17! + 17
17! = 1*2*3*...17
17!+2 = 2(1+1*3*..........17)
17! +2 is divisible by 2 .. not prime
Similarly check for 3,4,5....17
17! +3 is divisible by 3.. Not prime
17! +4 is divisible by 4..not prime
.......
....
17! +17 is divisible by 17.. not prime
Sufficient.
Pick B
--Anand--
- davedecibel
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you guys got it perfectly right, b is the correct answer, and the reasoning behind that is perfect.
i have some difficulties in such questions... do you think that manhattan "number properties" book would help me out on these topics?
i have some difficulties in such questions... do you think that manhattan "number properties" book would help me out on these topics?
- davedecibel
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the original explanation is pretty much the same you all figured out, it's just written badly imo O_ogoyalsau wrote: and please Daved , please share the Official explanation,
I am sure that will help a lot, till we don't have any other source of information..
- goyalsau
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selango wrote:
I there any way by which we can figure out Whether 17! + 1 is a prime number or not.
Saurabh Goyal
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- Brian@VeritasPrep
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Hey guys,
Great thread, and the answer is indeed B for the reasons everyone discussed. If you're interested, we actually had a great conversation about this one a few months ago that may have delved even further into it. Take a look:
https://www.beatthegmat.com/prime-numbers-t62545.html
Great thread, and the answer is indeed B for the reasons everyone discussed. If you're interested, we actually had a great conversation about this one a few months ago that may have delved even further into it. Take a look:
https://www.beatthegmat.com/prime-numbers-t62545.html
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
Looking for GMAT practice questions? Try out the Veritas Prep Question Bank. Learn More.
GMAT Instructor
Chief Academic Officer
Veritas Prep
Looking for GMAT practice questions? Try out the Veritas Prep Question Bank. Learn More.
- goyalsau
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Thanks Brian,Brian@VeritasPrep wrote:Hey guys,
Great thread, and the answer is indeed B for the reasons everyone discussed. If you're interested, we actually had a great conversation about this one a few months ago that may have delved even further into it. Take a look:
https://www.beatthegmat.com/prime-numbers-t62545.html
I got my answer.........
Saurabh Goyal
[email protected]
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EveryBody Wants to Win But Nobody wants to prepare for Win.
[email protected]
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EveryBody Wants to Win But Nobody wants to prepare for Win.