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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.
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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)
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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
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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
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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
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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.
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