What is the positive integer n?
(1) For every positive integer m, the product m(m + 1)(m + 2) ... (m + n) is divisible by 16
(2) n^2 - 9n + 20 = 0
OA in the source is C
But I think its E
positive integer n
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 118
- Joined: Mon May 21, 2012 10:07 pm
- Thanked: 23 times
- Followed by:4 members
- anuprajan5
- Master | Next Rank: 500 Posts
- Posts: 279
- Joined: Mon Jun 25, 2012 10:56 pm
- Thanked: 60 times
- Followed by:10 members
Ashmita,
The answer is C
Statement 1 - m(m=1)(m=2)....(m+n) is divisible by 16. Since these are consecutive integers, I only need a couple of even integers to make it divisible by 16.
For example, 1,2,3,4,5,6 - In this case n is 5
22,23,24 - In this case n is 2.
Insufficient
Statement 2 - This gives us 2 values of n ie: 5 and 4. Insufficient
Combining - we can find m is a series of consecutive integers where n is 5. Just to contradict the case, you can check if you can get a series of 5 consecutive numbers where n is 4. I didn't find one.
Regards
Anup
The answer is C
Statement 1 - m(m=1)(m=2)....(m+n) is divisible by 16. Since these are consecutive integers, I only need a couple of even integers to make it divisible by 16.
For example, 1,2,3,4,5,6 - In this case n is 5
22,23,24 - In this case n is 2.
Insufficient
Statement 2 - This gives us 2 values of n ie: 5 and 4. Insufficient
Combining - we can find m is a series of consecutive integers where n is 5. Just to contradict the case, you can check if you can get a series of 5 consecutive numbers where n is 4. I didn't find one.
Regards
Anup
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
Statement 1: For every positive integer m, the product m(m + 1)(m + 2) ... (m + n) is divisible by 16das.ashmita wrote:What is the positive integer n?
(1) For every positive integer m, the product m(m + 1)(m + 2) ... (m + n) is divisible by 16
(2) n^2 - 9n + 20 = 0
OA in the source is C
But I think its E
Since m is positive, m(m+1)(m+2)...(m+n) represents the product of CONSECUTIVE positive integers.
Every other consecutive EVEN integer is a MULTIPLE OF 4.
Thus, if the product includes 3 consecutive EVEN factors, there are two cases:
Case 1: (multiple of 2)(multiple of 4)(multiple of 2)
This product must be divisible by (2)(4)(2) = 16.
Case 2: (multiple of 4)(multiple of 2)(multiple of 4)
This product must be divisible by (4)(2)(4) = 32.
In each case, the product is divisible by 16.
Thus, if the product includes at least 3 consecutive even factors, it will be divisible by 16.
To GUARANTEE that the product will include at least 3 consecutive even factors, the MINIMUM value of n is 5:
m(m+1)(m+2)(m+3)(m+4)(m+5) = the product of 6 consecutive integers.
This product will be composed of exactly 3 consecutive odd factors and exactly 3 consecutive even factors.
If n=4 -- implying a total of 5 consecutive factors -- then the product could be (odd)(even)(odd)(even)(odd).
In this case, the product will not include at least 3 consecutive even factors, with the result that it might not be divisible by 16.
To illustrate:
Neither 1*2*3*4*5 nor 17*18*19*20*21 is divisible by 16.
Since the product must include at least 3 consecutive even factors to GUARANTEE that it will be divisible by 16, n≥5.
No way to determine the exact value of n.
INSUFFICIENT.
Statement 2: n² - 9n + 20 = 0
(n-4)(n-5) = 0.
Since it's possible that n=4 or n=5, INSUFFICIENT.
Statements 1 and 2 combined:
Only n=5 satisfies both statements.
SUFFICIENT.
The correct answer is C.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
GMAT/MBA Expert
- Anurag@Gurome
- GMAT Instructor
- Posts: 3835
- Joined: Fri Apr 02, 2010 10:00 pm
- Location: Milpitas, CA
- Thanked: 1854 times
- Followed by:523 members
- GMAT Score:770
Statement 1: Implies the product of any (n + 1) consecutive positive integers is divisible by 16. Now the product of any 6 or more consecutive integers is always divisible by 16. Hence, (n + 1) ≥ 6 => n ≥ 5das.ashmita wrote:What is the positive integer n?
(1) For every positive integer m, the product m(m + 1)(m + 2) ... (m + n) is divisible by 16
(2) n^2 - 9n + 20 = 0
OA in the source is C
But I think its E
Not sufficient
Statement 2: n² - 9n + 20 = 0
=> (n - 4)(n - 5) = 0
Hence, n = 4 or n = 5
Not sufficient
1 & 2 Together: As n ≥ 5, n must be equal to 5.
Sufficient
The correct answer is C.
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
-
- Master | Next Rank: 500 Posts
- Posts: 118
- Joined: Mon May 21, 2012 10:07 pm
- Thanked: 23 times
- Followed by:4 members
Thankyou Anurag , Mitch and Anup for replying.
I got confused in statement 1
case 1 : let m=2
2*3*4*5*6 is divisible by 16
Here n = 6-2 = 4
case 2 : let consider m=1
1*2*3*4*5*6 is divisible by 16
here n = 6-1 = 5
Can u please tell me what is wrong in assuming case 1
I got confused in statement 1
case 1 : let m=2
2*3*4*5*6 is divisible by 16
Here n = 6-2 = 4
case 2 : let consider m=1
1*2*3*4*5*6 is divisible by 16
here n = 6-1 = 5
Can u please tell me what is wrong in assuming case 1
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
You have cited ONE CASE in which n=4 yields a product divisible by 16.das.ashmita wrote:Thankyou Anurag , Mitch and Anup for replying.
I got confused in statement 1
case 1 : let m=2
2*3*4*5*6 is divisible by 16
Here n = 6-2 = 4
case 2 : let consider m=1
1*2*3*4*5*6 is divisible by 16
here n = 6-1 = 5
Can u please tell me what is wrong in assuming case 1
But statement 1 requires that the product be divisible by 16 FOR EVERY positive integer m.
If m=1 and n=4, the resulting product is 1*2*3*4*5, which is not divisible by 16.
Thus, n=4 does NOT satisfy the condition that the product be divisible by 16 FOR EVERY positive integer m.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
-
- Master | Next Rank: 500 Posts
- Posts: 118
- Joined: Mon May 21, 2012 10:07 pm
- Thanked: 23 times
- Followed by:4 members