• PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT
  • Varsity Tutors
    Award-winning private GMAT tutoring
    Register now and save up to $200

    Available with Beat the GMAT members only code

    MORE DETAILS
    Varsity Tutors
  • Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider
  • examPAL
    Most awarded test prep in the world
    Now free for 30 days

    Available with Beat the GMAT members only code

    MORE DETAILS
    examPAL
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep
  • e-gmat Exclusive Offer
    Get 300+ Practice Questions
    25 Video lessons and 6 Webinars for FREE

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep

question from Gmatprep test1

This topic has 3 member replies
banona Senior | Next Rank: 100 Posts Default Avatar
Joined
30 Dec 2006
Posted:
38 messages
Upvotes:
1

question from Gmatprep test1

Post Sat Mar 03, 2007 11:22 am
Can somebody tell me what the most efficient way to solve this kind of DS GMAT questions ;

the question says , if the remainder of the division of (n-1)*(n+1) by 24 is r, what's the value of r ?
1- n is not divisible by 2
2- n is not divisible by3

I appologize if the question rose somewhere else here in this forum while,butI didn't see it.

Thanks

  • +1 Upvote Post
  • Quote
  • Flag
rajesh_ctm Master | Next Rank: 500 Posts Default Avatar
Joined
27 Feb 2007
Posted:
135 messages
Upvotes:
2
Post Tue Apr 03, 2007 6:44 pm
jayhawk2001, that is a wonderful explanation! Thank you. Gives us a valuable technique, we can use for similar questions.

  • +1 Upvote Post
  • Quote
  • Flag
banona Senior | Next Rank: 100 Posts Default Avatar
Joined
30 Dec 2006
Posted:
38 messages
Upvotes:
1
Post Mon Mar 05, 2007 5:50 am
thank you,
I would go for picking numbers on the real GMAT too.

  • +1 Upvote Post
  • Quote
  • Flag
jayhawk2001 Community Manager
Joined
28 Jan 2007
Posted:
789 messages
Followed by:
1 members
Upvotes:
30
Post Sun Mar 04, 2007 6:36 pm
Not sure if this is the most efficient way but here goes...

1 - insufficient. Take n = 1 and n = 3 for example.
They yield remainders of 0 and 8 resp. No unique value. So rule out 1.

2 - insufficient. Take n = 1 and n = 2.
They yield remainders of 0 and 3 resp. No unique value. So, rule out 2

A number that is not divisible by 2 AND not divisible by 3 can be
written as n = 6x + 1 or n = 6x - 1

For n = 6x+1
(n-1)*(n+1) / 24
= 6x * (6x+2) / 24
= 12x * (x+1) / 24

We know that x*(x+1) will be even or 0, so the above equation will
always yield a remainder of 0.

Similarly for n=6x-1
(n-1)*(n+1) / 24
= (6x - 2)*6x
= 12x*(x-1) / 24

Again, x*x(-1) will be even or 0 and hence remainder r will be 0.

So, C looks like the correct answer.

On the actual GMAT, in the interest of time, it might be worthwhile
trying n = 1, 3, 5, 7, 9 etc. We can see that the remainder is 8 when n
is not divisible by 3 and remainder is 0 when n is divisible by 3 (for
all values n > 0). Using this pattern, I guess we can settle on C.

  • +1 Upvote Post
  • Quote
  • Flag

Best Conversation Starters

1 lheiannie07 108 topics
2 Roland2rule 63 topics
3 ardz24 63 topics
4 LUANDATO 50 topics
5 AAPL 42 topics
See More Top Beat The GMAT Members...

Most Active Experts

1 image description GMATGuruNY

The Princeton Review Teacher

152 posts
2 image description Jeff@TargetTestPrep

Target Test Prep

106 posts
3 image description Rich.C@EMPOWERgma...

EMPOWERgmat

104 posts
4 image description Scott@TargetTestPrep

Target Test Prep

96 posts
5 image description Max@Math Revolution

Math Revolution

87 posts
See More Top Beat The GMAT Experts