BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Is the answer given wrong?

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 80
Joined: Mon Feb 02, 2009 6:36 am
Thanked: 10 times

Is the answer given wrong?

by billzhao » Mon Feb 02, 2009 3:56 pm
Problem: The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

The answer to the question is n=79.

but my calculation derives n = 159 based on S=a*(n-1)/2 + 1/2*(n-1)/2*((n-1)/2-1)*d = 79*80, where a=2 and d=2.

Any one can help?

Thanks!
Yiliang
Top
Source: — Problem Solving |
User avatar
Site Admin
Posts: 2567
Joined: Thu Jan 01, 2009 10:05 am
Thanked: 712 times
Followed by:550 members
GMAT Score:770

by DanaJ » Tue Feb 03, 2009 7:23 am
https://www.beatthegmat.com/tricky-one-t22481.html
Top
Senior | Next Rank: 100 Posts
Posts: 45
Joined: Sun Nov 30, 2008 8:41 pm
Thanked: 4 times

by mrsmarthi » Tue Feb 03, 2009 7:44 pm
We have rule to remember

Sum of first X even integers = X * (X + 1)

i.e 2 + 4 + 6 + ......2X = X*(X+1) or
2(1 + 2 + 3 ....+ x) = X*(x+1)

Given that n is odd. Hence n-1 should be the last even number.

Given, 2 + 4 + 6 +......(n-1)= 79 * 80.

i.e we have in total (n-1)/2 terms.

Substituing the above rule, we have (n-1)/2 * ((n-1)/2) + 1)= 79 * 80.

(n-1)/2 = 79 ==> n = 159.

I would go with N = 159.
Top
Senior | Next Rank: 100 Posts
Posts: 80
Joined: Mon Feb 02, 2009 6:36 am
Thanked: 10 times

Thanks for your help!

by billzhao » Tue Feb 03, 2009 8:14 pm
Thanks for your help!
Yiliang
Top
Post new topic Post Reply
• Page 1 of 1