• Get 300+ Practice Questions

Available with Beat the GMAT members only code

• 1 Hour Free
BEAT THE GMAT EXCLUSIVE

Available with Beat the GMAT members only code

• Free Veritas GMAT Class
Experience Lesson 1 Live Free

Available with Beat the GMAT members only code

• 5 Day FREE Trial
Study Smarter, Not Harder

Available with Beat the GMAT members only code

• 5-Day Free Trial
5-day free, full-access trial TTP Quant

Available with Beat the GMAT members only code

• Free Trial & Practice Exam
BEAT THE GMAT EXCLUSIVE

Available with Beat the GMAT members only code

• Magoosh
Study with Magoosh GMAT prep

Available with Beat the GMAT members only code

• Award-winning private GMAT tutoring
Register now and save up to $200 Available with Beat the GMAT members only code • Free Practice Test & Review How would you score if you took the GMAT Available with Beat the GMAT members only code • FREE GMAT Exam Know how you'd score today for$0

Available with Beat the GMAT members only code

## Difficult Math Problem #62 - Algebra

This topic has 2 member replies
800guy Master | Next Rank: 500 Posts
Joined
27 Jun 2006
Posted:
354 messages
Followed by:
5 members
11

#### Difficult Math Problem #62 - Algebra

Fri Nov 24, 2006 11:24 am
The sum of the even numbers between 1 and n is 79*80, where n is an odd number. n=?

gmat_enthus Senior | Next Rank: 100 Posts
Joined
18 Oct 2006
Posted:
62 messages
1
Fri Nov 24, 2006 12:35 pm
800guy wrote:
The sum of the even numbers between 1 and n is 79*80, where n is an odd number. n=?
sum of even numbers = n1/2 ( 4 + (n1-1)2) = 79*80

n1(2+2n1)/2 = 79*80
n1(n1+1) = 79*80

=> n1=79. So 79 even numbers => nth term = 2 + 78*2 = 158

So "n" in the question should be 159

_________________
My 760 | GMAT Preparation Suggestion

destination 760! completed!

800guy Master | Next Rank: 500 Posts
Joined
27 Jun 2006
Posted:
354 messages
Followed by:
5 members
11
Mon Nov 27, 2006 12:46 pm
OA:

The sum of numbers between 1 and n is = (n(n+1))/2

1+2+3+.....+n=(n(n+1))/2 {formula}

we are looking for the sum of the even numbers between 1 and n, which means:

2+4+6+.....+(n-1) n is ODD
=1*2+2*2+2*3+......+2*((n-1)/2)
=2*(1+2+3+.....+*((n-1)/2))
from the formula we obtain :
=2*(((n-1)/2)*((n-1)/2+1))/2
=((n-1)/2)*((n+1)/2) =79*80
=> (n-1)*(n+1)=158*160
=> n=159

### Top First Responders*

1 GMATGuruNY 68 first replies
2 Rich.C@EMPOWERgma... 48 first replies
3 Brent@GMATPrepNow 37 first replies
4 Jay@ManhattanReview 26 first replies
5 ErikaPrepScholar 9 first replies
* Only counts replies to topics started in last 30 days
See More Top Beat The GMAT Members

### Most Active Experts

1 GMATGuruNY

The Princeton Review Teacher

128 posts
2 Rich.C@EMPOWERgma...

EMPOWERgmat

117 posts
3 Jeff@TargetTestPrep

Target Test Prep

106 posts
4 Max@Math Revolution

Math Revolution

94 posts
5 Scott@TargetTestPrep

Target Test Prep

92 posts
See More Top Beat The GMAT Experts