Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 01 Jul 2016, 00:05

### 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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# The sum of the even numbers between 1 and n is 79*80, where

 post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
Senior Manager
Joined: 14 Jul 2006
Posts: 281
Followers: 1

Kudos [?]: 0 [0], given: 0

The sum of the even numbers between 1 and n is 79*80, where [#permalink]

### Show Tags

18 Aug 2006, 02:34
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

The sum of the even numbers between 1 and n is 79*80, where n is an odd number. N=?

HI guys I'm looking for a faster way to solve this problem.

I just got the range from 2 to n-1. Obtained the average (n-1+2)2
multiplied it by the number of terms (n-1)/2, and equated it to 79 and 80.
Can anyone suggest a faster way of arriving at the answer? I think the method that I used is time consuming.
GMAT Instructor
Joined: 04 Jul 2006
Posts: 1264
Location: Madrid
Followers: 27

Kudos [?]: 245 [0], given: 0

Re: question bank problem solving [#permalink]

### Show Tags

18 Aug 2006, 03:25
apollo168 wrote:
The sum of the even numbers between 1 and n is 79*80, where n is an odd number. N=?

HI guys I'm looking for a faster way to solve this problem.

I just got the range from 2 to n-1. Obtained the average (n-1+2)2
multiplied it by the number of terms (n-1)/2, and equated it to 79 and 80.
Can anyone suggest a faster way of arriving at the answer? I think the method that I used is time consuming.

I know know whether this is any better, but here it goes:

If n is an odd number (obviously positive) we can write n=2k+1 where k is a positive integer. The greatest even number less than n is 2k.

So the sum of the positive even numbers less than n= 2+4+6+...+2k =2(1+2+3....+k)=2(k+1)k/2 =(k)(k+1)=79(80)

Thus k=79 and n=2(79)+1=159 This took about 1 minute
Senior Manager
Joined: 14 Jul 2006
Posts: 281
Followers: 1

Kudos [?]: 0 [0], given: 0

[#permalink]

### Show Tags

18 Aug 2006, 03:39
Hi kevin,

Certainly looks more efficient. But can't seem to decipher this part of your solution

2(k+1)k/2 --> I know that the average of the evenly spaced even numbers is 2(k+1)/2 ...but what I don't know is why you just multiplied the average by k? Won't multiplying the average by k also include the odd numbers? Sorry to inconvenience you again.
Senior Manager
Joined: 14 Jul 2006
Posts: 281
Followers: 1

Kudos [?]: 0 [0], given: 0

[#permalink]

### Show Tags

18 Aug 2006, 03:43
Sorry tried it out again. I got it!

Thanks
[#permalink] 18 Aug 2006, 03:43
Display posts from previous: Sort by

# The sum of the even numbers between 1 and n is 79*80, where

 post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group and phpBB SEO 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®.