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

It is currently 17 Jun 2018, 16:56

Close

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
Your Progress

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

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Close

Request Expert Reply

Confirm Cancel

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

If the sum of the first k positive integers is equal to, k(k+1)/2, Wha

  post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Expert Post
Math Revolution GMAT Instructor
User avatar
D
Joined: 16 Aug 2015
Posts: 5562
GMAT 1: 800 Q59 V59
GPA: 3.82
If the sum of the first k positive integers is equal to, k(k+1)/2, Wha [#permalink]

Show Tags

New post Updated on: 27 Sep 2017, 03:21
00:00
A
B
C
D
E

Difficulty:

  65% (hard)

Question Stats:

47% (01:33) correct 53% (00:54) wrong based on 76 sessions

HideShow timer Statistics

If the sum of the first k positive integers is equal to, k(k+1)/2, What is the sum of the integers from n to m, inclusive, where 0<n<m?

A. (m(m+1)-(n-1)n)/2
B. (m(m+1)-(n+1)n)/2
C. (m(m-1)-(n-1)n)/2
D. (m(m-1)-(n+1)n)/2
E. (m(m+1)-(n-1)n)/4


* A solution will be posted in two days.

OPEN DISCUSSION OF THIS QUESTION IS HERE: https://gmatclub.com/forum/the-sum-of-t ... 26289.html

_________________

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 $79 for 3 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"


Originally posted by MathRevolution on 25 Jan 2016, 17:40.
Last edited by Bunuel on 27 Sep 2017, 03:21, edited 3 times in total.
Added the OA.
Manager
Manager
avatar
S
Joined: 12 Mar 2015
Posts: 100
Concentration: Leadership, Finance
GPA: 3.9
WE: Information Technology (Computer Software)
Re: If the sum of the first k positive integers is equal to, k(k+1)/2, Wha [#permalink]

Show Tags

New post 26 Jan 2016, 01:06
MathRevolution wrote:
If the sum of the first k positive integers is equal to, k(k+1)/2, What is the sum of the integers from n to m, inclusive, where 0<n<m?

A. (m(m+1)-(n-1)n)/2
B. (m(m+1)-(n+1)n)/2
C. (m(m-1)-(n-1)n)/2
D. (m(m-1)-(n+1)n)/2
E. (m(m+1)-(n-1)n)/4


* A solution will be posted in two days.



1+2+3+4..+n+...+m

m(m+1)/2 - (n-1)(n-1+1)/2
= (m(m+1) - (n-1)(n))/2

Answer A
Expert Post
1 KUDOS received
Math Revolution GMAT Instructor
User avatar
D
Joined: 16 Aug 2015
Posts: 5562
GMAT 1: 800 Q59 V59
GPA: 3.82
Re: If the sum of the first k positive integers is equal to, k(k+1)/2, Wha [#permalink]

Show Tags

New post 27 Jan 2016, 18:23
1
If the sum of the first k positive integers is equal to, k(k+1)/2, What is the sum of the integers from n to m, inclusive, where 0<n<m?

A. (m(m+1)-(n-1)n)/2
B. (m(m+1)-(n+1)n)/2
C. (m(m-1)-(n-1)n)/2
D. (m(m-1)-(n+1)n)/2
E. (m(m+1)-(n-1)n)/4


==> n+(n+1)+…….+(m-1)+m={1+2+…+(n-1)+n+(n+1)+…+(m-1)+m}-{1+2+…+(n-1)}=m(m+1)/2-n(n-1)/2.

Therefore, the answer is A.
_________________

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 $79 for 3 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"

1 KUDOS received
SC Moderator
avatar
D
Joined: 22 May 2016
Posts: 1743
Premium Member CAT Tests
Re: If the sum of the first k positive integers is equal to, k(k+1)/2, Wha [#permalink]

Show Tags

New post 26 Aug 2017, 12:01
1
MathRevolution wrote:
If the sum of the first k positive integers is equal to, k(k+1)/2, What is the sum of the integers from n to m, inclusive, where 0<n<m?

A. (m(m+1)-(n-1)n)/2
B. (m(m+1)-(n+1)n)/2
C. (m(m-1)-(n-1)n)/2
D. (m(m-1)-(n+1)n)/2
E. (m(m+1)-(n-1)n)/4


* A solution will be posted in two days.

If you don't know how to manipulate that particular formula, or if the manipulation takes too long, choose values.

Let n = 2 and m = 4

Find the sum of the integers from n to m, inclusive. Either

1) 2 + 3 + 4 = 9, or

2) [IF you choose numbers too far apart]: (average) * (# of terms) is

\(\frac{(2+4)}{2}\) * 3 = 9

Check answer choices with n = 2, m = 4. The one that yields 9 as the answer is correct.

The arithmetic with these numbers is quick.

Answer A. (m(m+1)-(n-1)n)/2
(4*5 - 1*2)/2 = 18/2 = 9. MATCH

I checked the others, quickly.

B. (m(m+1)-(n+1)n)/2
(20 - 6)/2 = 7. NOT a match

C. (m(m-1)-(n-1)n)/2
(12 - 1) = 11 won't work as a numerator. Move on. NOT a match

D. (m(m-1)-(n+1)n)/2
(12 - 6) = 6, already too small without division. Move on. NOT a match.

E. (m(m+1)-(n-1)n)/4
(20 - 1) = 19 won't work as a numerator. Move on. NOT a match

Answer A
_________________

In the depths of winter, I finally learned
that within me there lay an invincible summer.

-- Albert Camus, "Return to Tipasa"

Expert Post
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 46035
Re: If the sum of the first k positive integers is equal to, k(k+1)/2, Wha [#permalink]

Show Tags

New post 27 Sep 2017, 03:21
MathRevolution wrote:
If the sum of the first k positive integers is equal to, k(k+1)/2, What is the sum of the integers from n to m, inclusive, where 0<n<m?

A. (m(m+1)-(n-1)n)/2
B. (m(m+1)-(n+1)n)/2
C. (m(m-1)-(n-1)n)/2
D. (m(m-1)-(n+1)n)/2
E. (m(m+1)-(n-1)n)/4


* A solution will be posted in two days.


OPEN DISCUSSION OF THIS QUESTION IS HERE: https://gmatclub.com/forum/the-sum-of-t ... 26289.html
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
Extra-hard Quant Tests with Brilliant Analytics

Re: If the sum of the first k positive integers is equal to, k(k+1)/2, Wha   [#permalink] 27 Sep 2017, 03:21
Display posts from previous: Sort by

If the sum of the first k positive integers is equal to, k(k+1)/2, Wha

  post reply Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Terms and Conditions and Privacy Policy| GMAT Club Rules| Contact| Sitemap

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®.