BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Number system

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Legendary Member
Posts: 1132
Joined: Mon Jul 20, 2009 3:38 am
Location: India
Thanked: 64 times
Followed by:6 members
GMAT Score:760

Number system

by harsh.champ » Tue Feb 09, 2010 4:02 am
The number of positive integers n in the range 12 < n< 40 such that the product (n - 1) (n -2)...3.2.1 is not divisible by n is

(A)5
(B)7
(C)13
(D)14
(E)9

The OA is B.
It takes time and effort to explain, so if my comment helped you please press Thanks button :)



Just because something is hard doesn't mean you shouldn't try,it means you should just try harder.

"Keep Walking" - Johnny Walker :P
Top
Source: — Problem Solving |
User avatar
Community Manager
Posts: 1537
Joined: Mon Aug 10, 2009 6:10 pm
Thanked: 653 times
Followed by:252 members

by papgust » Tue Feb 09, 2010 4:08 am
The question basically asks for the prime numbers b/w 12 and 40 exclusive. No. of prime numbers that fall under this condition is 7 (B)
Top
User avatar
Legendary Member
Posts: 1022
Joined: Mon Jul 20, 2009 11:49 pm
Location: Gandhinagar
Thanked: 41 times
Followed by:2 members

by shashank.ism » Tue Feb 09, 2010 4:42 am
papgust wrote:The question basically asks for the prime numbers b/w 12 and 40 exclusive. No. of prime numbers that fall under this condition is 7 (B)
Papgust how did you thought of prime numbers being only asked here..Though your soln is very correct, but I want to know whether its applicable if we choose some other range..
If it is true surely we can easily calculate for big range too without any fuzz... like for range 11<x<8934.
please explain
My Websites:
www.mba.webmaggu.com - India's social Network for MBA Aspirants

www.deal.webmaggu.com -India's online discount, coupon, free stuff informer.

www.dictionary.webmaggu.com - A compact free online dictionary with images.

Nothing is Impossible, even Impossible says I'm possible.
Top
User avatar
Legendary Member
Posts: 1275
Joined: Thu Sep 21, 2006 11:13 pm
Location: Arabian Sea
Thanked: 125 times
Followed by:2 members

by ajith » Tue Feb 09, 2010 5:55 am
harsh.champ wrote:The number of positive integers n in the range 12 < n< 40 such that the product (n - 1) (n -2)...3.2.1 is not divisible by n is

(A)5
(B)7
(C)13
(D)14
(E)9

The OA is B.
n should be a prime to satisfy this criterion
The primes between 12and 40 are
13,17,19,23,29,31,37 - 7 of them
B
Always borrow money from a pessimist, he doesn't expect to be paid back.
Top
User avatar
Legendary Member
Posts: 1275
Joined: Thu Sep 21, 2006 11:13 pm
Location: Arabian Sea
Thanked: 125 times
Followed by:2 members

by ajith » Tue Feb 09, 2010 5:59 am
shashank.ism wrote:
papgust wrote:The question basically asks for the prime numbers b/w 12 and 40 exclusive. No. of prime numbers that fall under this condition is 7 (B)
Papgust how did you thought of prime numbers being only asked here..Though your soln is very correct, but I want to know whether its applicable if we choose some other range..
If it is true surely we can easily calculate for big range too without any fuzz... like for range 11<x<8934.
please explain
Only prime numbers follow this property

Say a number n, which has two factors k and p; k, p both will be covered in 1*2*....*n-1 and hence 1*2*....*n-1 will be divisible by n

and if it was higher range you would have to find all the prime numbers, but let us assume that GMAC is sane
Always borrow money from a pessimist, he doesn't expect to be paid back.
Top
User avatar
Legendary Member
Posts: 1022
Joined: Mon Jul 20, 2009 11:49 pm
Location: Gandhinagar
Thanked: 41 times
Followed by:2 members

by shashank.ism » Tue Feb 09, 2010 11:39 am
ajith wrote:
shashank.ism wrote:
papgust wrote:The question basically asks for the prime numbers b/w 12 and 40 exclusive. No. of prime numbers that fall under this condition is 7 (B)
Papgust how did you thought of prime numbers being only asked here..Though your soln is very correct, but I want to know whether its applicable if we choose some other range..
If it is true surely we can easily calculate for big range too without any fuzz... like for range 11<x<8934.
please explain
Only prime numbers follow this property

Say a number n, which has two factors k and p; k, p both will be covered in 1*2*....*n-1 and hence 1*2*....*n-1 will be divisible by n

and if it was higher range you would have to find all the prime numbers, but let us assume that GMAC is sane
Hmm that is good then..
yeah i was just assuming and I knoe GMAT will not give such a big range I was just trying to snatch out any other approach i available from u guys ... something called brahmashtra.
My Websites:
www.mba.webmaggu.com - India's social Network for MBA Aspirants

www.deal.webmaggu.com -India's online discount, coupon, free stuff informer.

www.dictionary.webmaggu.com - A compact free online dictionary with images.

Nothing is Impossible, even Impossible says I'm possible.
Top
Post new topic Post Reply
• Page 1 of 1