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

 It is currently 17 Feb 2019, 18:35

### 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

## Events & Promotions

###### Events & Promotions in February
PrevNext
SuMoTuWeThFrSa
272829303112
3456789
10111213141516
17181920212223
242526272812
Open Detailed Calendar
• ### Free GMAT Algebra Webinar

February 17, 2019

February 17, 2019

07:00 AM PST

09:00 AM PST

Attend this Free Algebra Webinar and learn how to master Inequalities and Absolute Value problems on GMAT.
• ### Valentine's day SALE is on! 25% off.

February 18, 2019

February 18, 2019

10:00 PM PST

11:00 PM PST

We don’t care what your relationship status this year - we love you just the way you are. AND we want you to crush the GMAT!

# In a certain conference, if all the n attendees shake hands 105 times,

Author Message
TAGS:

### Hide Tags

Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 6955
GMAT 1: 760 Q51 V42
GPA: 3.82
In a certain conference, if all the n attendees shake hands 105 times,  [#permalink]

### Show Tags

18 Aug 2017, 00:56
00:00

Difficulty:

25% (medium)

Question Stats:

77% (01:32) correct 23% (01:20) wrong based on 137 sessions

### HideShow timer Statistics

In a certain conference, if all the n attendees shake hands 105 times, what is the value of n?

A. 10
B. 12
C. 15
D. 16
E. 18

_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $149 for 3 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Senior Manager Joined: 19 Apr 2016 Posts: 274 Location: India GMAT 1: 570 Q48 V22 GMAT 2: 640 Q49 V28 GPA: 3.5 WE: Web Development (Computer Software) Re: In a certain conference, if all the n attendees shake hands 105 times, [#permalink] ### Show Tags 18 Aug 2017, 01:01 1 MathRevolution wrote: In a certain conference, if all the n attendees shake hands 105 times, what is the value of n? A. 10 B. 12 C. 15 D. 16 E. 18 n(n-1)/2 = 105 n(n-1) = 210 (n-15)(n+14) = 210 n = 15 Hence option C is correct Hit Kudos if you liked it 8-) Manager Joined: 27 Dec 2016 Posts: 233 Concentration: Marketing, Social Entrepreneurship GPA: 3.65 WE: Marketing (Education) Re: In a certain conference, if all the n attendees shake hands 105 times, [#permalink] ### Show Tags 18 Aug 2017, 03:39 I solved this question in 1,5 minutes, but with plug in method. Since this one is a combination problem, so we should determine what nCr will have 105 in result. MathRevolution, do we have more straightforward approach? _________________ There's an app for that - Steve Jobs. Math Expert Joined: 02 Aug 2009 Posts: 7334 Re: In a certain conference, if all the n attendees shake hands 105 times, [#permalink] ### Show Tags 18 Aug 2017, 04:02 MathRevolution wrote: In a certain conference, if all the n attendees shake hands 105 times, what is the value of n? A. 10 B. 12 C. 15 D. 16 E. 18 Hi.. Ofcourse the Q should mention that each shake hands with others once. So choosing 2 out of n is nC2=$$\frac{n!}{(n-2)!2!}=\frac{n*(n-1)*(n-2)!}{(n-2)!*2!}=\frac{n*(n-1)}{2}=105....$$... $$n(n-1)=210=2*3*5*7=3*5*2*7=15*14$$ So n is 15 C _________________ 1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372 2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html 3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html 4) Base while finding % increase and % decrease : https://gmatclub.com/forum/percentage-increase-decrease-what-should-be-the-denominator-287528.html GMAT Expert Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 6955 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: In a certain conference, if all the n attendees shake hands 105 times, [#permalink] ### Show Tags 20 Aug 2017, 18:14 nC2 = 105. n(n-1)/(1*2) = 105 n(n-1) = 210 n(n-1) = 15*14 Thus n = 15. Ans: C _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$149 for 3 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"

Director
Joined: 19 Oct 2013
Posts: 508
Location: Kuwait
GPA: 3.2
WE: Engineering (Real Estate)
Re: In a certain conference, if all the n attendees shake hands 105 times,  [#permalink]

### Show Tags

13 Oct 2018, 03:28
MathRevolution wrote:
In a certain conference, if all the n attendees shake hands 105 times, what is the value of n?

A. 10
B. 12
C. 15
D. 16
E. 18

Backsolving

Nc2 = (n)(n-1)/2

Pick a number and try.

(15 * 14)/2 = 15 * 7 = 105.

Posted from my mobile device
Re: In a certain conference, if all the n attendees shake hands 105 times,   [#permalink] 13 Oct 2018, 03:28
Display posts from previous: Sort by