GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 17 Aug 2018, 13:53

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

If n is a positive integer, is 1/n - 1/(n+1) <0.05? (1) n is odd (2

Author Message
TAGS:

Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 47977
If n is a positive integer, is 1/n - 1/(n+1) <0.05? (1) n is odd (2  [#permalink]

Show Tags

01 Aug 2018, 05:16
00:00

Difficulty:

35% (medium)

Question Stats:

68% (01:51) correct 32% (01:21) wrong based on 50 sessions

HideShow timer Statistics

If n is a positive integer, is $$\frac{1}{n} - \frac{1}{n+1} <0.05$$?

(1) n is odd

(2) n is a positive multiple of 7

_________________
Manager
Joined: 17 Jan 2017
Posts: 137
Location: India
GPA: 4
WE: Information Technology (Computer Software)
Re: If n is a positive integer, is 1/n - 1/(n+1) <0.05? (1) n is odd (2  [#permalink]

Show Tags

01 Aug 2018, 05:40
Statement 1:
N = 1
N+1 = 2
1/(n(n+1)) = 1/2 < 0.05 (false)
Or
N=19
N+1=20
1/(n(n+1)) = 1/380 < 0.05 (true)
Insufficient.

Statement 2:
N is a positive multiple of 7.
Lowest value of n can be assumed to be N=7
N+1=8
1/(n(n+1)) = 1/56 < 0.05 (true)
Sufficient

Hence B

Posted from my mobile device
_________________

Correct me If I'm wrong !! looking for valuable inputs

KUDOS please, if you like the post or if it helps !!

Director
Joined: 30 Jan 2015
Posts: 525
Location: India
Concentration: Operations, Marketing
GPA: 3.5
Re: If n is a positive integer, is 1/n - 1/(n+1) <0.05? (1) n is odd (2  [#permalink]

Show Tags

01 Aug 2018, 05:53
Bunuel wrote:
If n is a positive integer, is $$\frac{1}{n} - \frac{1}{n+1} <0.05$$?

​1 / n - 1 / (n+1) < 0.05
(n + 1 - n) / n(n+1) < 0.05
1 / n(n+1) < 0.05
1 / n(n+1) < 1 / 20
n(n+1) > 20 -- A)

(1) n is odd
Plugging values in n(n+1) > 20, multiple values both less than and greater than 20 are possible
Insufficient

(2) n is a positive multiple of 7
Plugging values in n(n+1) > 20
7 * (7+1) = 7 * 8 > 20
Since the least value obtained is greater than 20, its is always greater than 20
Sufficient

Hence, B.
_________________

The few, the fearless !

Thanks

Manager
Joined: 18 Jun 2018
Posts: 102
Re: If n is a positive integer, is 1/n - 1/(n+1) <0.05? (1) n is odd (2  [#permalink]

Show Tags

01 Aug 2018, 06:07
Bunuel wrote:
If n is a positive integer, is $$\frac{1}{n} - \frac{1}{n+1} <0.05$$?

(1) n is odd

(2) n is a positive multiple of 7

OA: B
n>0, n is an integer
Is $$\frac{1}{n} - \frac{1}{n+1} <0.05$$
Simplifying the question stem
$$\frac{1}{n} - \frac{1}{n+1} <0.05$$
$$\frac{(n+1)-(n)}{n(n+1)}< \frac{5}{100}$$
$$\frac{1}{n(n+1)}< \frac{1}{20}$$
As $$n$$ is positive, multiplying both sides by $$20n(n+1)$$, we get
$$20<n(n+1)$$
$$n(n+1)-20>0$$
$$n^2+n-20>0$$
$$(n+5)(n-4)>0$$
This leads to $$n<-5$$ or $$n>4.$$
as n is positive,Question stem reduces to Is n>4?

Statement 1: n is odd
For n=1,3
Is n>4? : No
For n =5,7,9......
Is n>4? : Yes
Statement 1 alone is insufficient

Statement 2 :n is a positive multiple of 7
Minimum value of n is 7, as it is first positive multiple of 7
Is n>4? : Yes
Statement 2 alone is sufficient
Intern
Joined: 23 Aug 2017
Posts: 25
Re: If n is a positive integer, is 1/n - 1/(n+1) <0.05? (1) n is odd (2  [#permalink]

Show Tags

01 Aug 2018, 13:44
Bunuel wrote:
If n is a positive integer, is $$\frac{1}{n} - \frac{1}{n+1} <0.05$$?

(1) n is odd

(2) n is a positive multiple of 7

Simplifying the stem:
$$\frac{1}{n} - \frac{1}{n+1} <0.05$$
$$\frac{1}{n(n+1)} < \frac{5}{100}$$
$$\frac{100}{n(n+1)} < 5$$

1) Not possible to solve only konwing N is odd or even. Not sufficient.
2) If we replaced N for any multiple of 7 the result will always be less than 5. Sufficient.

Option B.
Re: If n is a positive integer, is 1/n - 1/(n+1) <0.05? (1) n is odd (2 &nbs [#permalink] 01 Aug 2018, 13:44
Display posts from previous: Sort by

Events & Promotions

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.