NEW HERE? LEARN MORE
closeclose
Last visit was: 23 Apr 2024, 10:31 It is currently 23 Apr 2024, 10:31
Decision Tracker
My Rewards
New posts
New comers' posts

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.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel

PS-Prep, time killer

User avatar
misterJJ2u
Manager
Manager
Joined: 29 Nov 2006
Posts: 172
Own Kudos [?]: 1237 [0]
Given Kudos: 0
Location: Orange County, CA
Send PM
PS-Prep, time killer [#permalink] New post   28 Jun 2007, 09:19
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
Hide Show timer Statistics
Any suggestions for a quicker answer?

I was taking too long determining sign changes and adding / subtracting between each item (1-10) and ended up just guessing in order to move onto the next problem.

Looking for ways to cut down time on easy questions like this.



Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Problem Solving (PS) Forum
Still interested in this question? Check out the "Best Topics" block below for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
User avatar
KillerSquirrel
Director
Director
Joined: 08 Jun 2005
Posts: 523
Own Kudos [?]: 556 [0]
Given Kudos: 0
Send PM
Re: PS-Prep, time killer [#permalink] New post   28 Jun 2007, 10:38
using Geometric Progression formula is shorter but easy to make mistakes:

Sn = a(1-R^n)/(1-R)

n = 10
a = 1/2

F(1) = 1/2
F(2) = -1/4
F(3) = 1/8

hence R = -1/2

solve GP

Sn = 1/2*(1-(-1/2^10))/(1-(-1/2))

Sn = (1025/2048)/(3/2)

Sn = 2050/2048/3

Sn = ~1/3

the answer is (D)

:-D
User avatar
ian7777
CIO
CIO
Joined: 09 Mar 2003
Posts: 230
Own Kudos [?]: 171 [0]
Given Kudos: 0
Send PM
Re: PS-Prep, time killer [#permalink] New post   28 Jun 2007, 12:06
misterJJ2u wrote:
Any suggestions for a quicker answer?

I was taking too long determining sign changes and adding / subtracting between each item (1-10) and ended up just guessing in order to move onto the next problem.

Looking for ways to cut down time on easy questions like this.


Also D. 1 minute.

1. Find pattern. There has to be one, otherwise it wouldn't be a GMAT question. Very quickly see that this thing is just alternating positive and negative powers of 1/2:
1/2, -1/4, 1/8, -1/16....

2. Every change is smaller than the previous one. Classic GMAT game. I drew a number line:

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2

Numbers start at 1/2, then down 1/4, then up 1/8, then down 1/16. You basically bounce back and fourth, but you NEVER cross 1/4, and you never cross 1/2.

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2
...................................................... Start
............................ {---------------------- ....(Back 1/4)
............................. ------------} ...............(forward 1/8)
......................................{----- ................(back 1/16)
.......................................---} .................(forward 1/32)
....................................... {- .................. (back 1/64)

Etc, etc. It's a lot here to type out, on paper, really simple. Either way, you can see the range you're in pretty clearly.
User avatar
KillerSquirrel
Director
Director
Joined: 08 Jun 2005
Posts: 523
Own Kudos [?]: 556 [0]
Given Kudos: 0
Send PM
Re: PS-Prep, time killer [#permalink] New post   28 Jun 2007, 12:25
ian7777 wrote:
misterJJ2u wrote:
Any suggestions for a quicker answer?

I was taking too long determining sign changes and adding / subtracting between each item (1-10) and ended up just guessing in order to move onto the next problem.

Looking for ways to cut down time on easy questions like this.


Also D. 1 minute.

1. Find pattern. There has to be one, otherwise it wouldn't be a GMAT question. Very quickly see that this thing is just alternating positive and negative powers of 1/2:
1/2, -1/4, 1/8, -1/16....

2. Every change is smaller than the previous one. Classic GMAT game. I drew a number line:

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2

Numbers start at 1/2, then down 1/4, then up 1/8, then down 1/16. You basically bounce back and fourth, but you NEVER cross 1/4, and you never cross 1/2.

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2
...................................................... Start
............................ {---------------------- ....(Back 1/4)
............................. ------------} ...............(forward 1/8)
......................................{----- ................(back 1/16)
.......................................---} .................(forward 1/32)
....................................... {- .................. (back 1/64)

Etc, etc. It's a lot here to type out, on paper, really simple. Either way, you can see the range you're in pretty clearly.


Nice ! - Thanks for your help - I'm guessing its a lot of work to write it all down. A true and elegant solution.

:)
User avatar
Himalayan
Senior Manager
Senior Manager
Joined: 26 Feb 2006
Posts: 384
Own Kudos [?]: 583 [0]
Given Kudos: 0
Send PM
Re: PS-Prep, time killer [#permalink] New post   28 Jun 2007, 12:33
ian7777 wrote:
misterJJ2u wrote:
Any suggestions for a quicker answer?

I was taking too long determining sign changes and adding / subtracting between each item (1-10) and ended up just guessing in order to move onto the next problem.

Looking for ways to cut down time on easy questions like this.


Also D. 1 minute.

1. Find pattern. There has to be one, otherwise it wouldn't be a GMAT question. Very quickly see that this thing is just alternating positive and negative powers of 1/2:
1/2, -1/4, 1/8, -1/16....

2. Every change is smaller than the previous one. Classic GMAT game. I drew a number line:

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2

Numbers start at 1/2, then down 1/4, then up 1/8, then down 1/16. You basically bounce back and fourth, but you NEVER cross 1/4, and you never cross 1/2.

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2
...................................................... Start
............................ {---------------------- ....(Back 1/4)
............................. ------------} ...............(forward 1/8)
......................................{----- ................(back 1/16)
.......................................---} .................(forward 1/32)
....................................... {- .................. (back 1/64)

Etc, etc. It's a lot here to type out, on paper, really simple. Either way, you can see the range you're in pretty clearly.


wonderful approach..


btw, i got 341/1024 using my lengthy approach..
User avatar
misterJJ2u
Manager
Manager
Joined: 29 Nov 2006
Posts: 172
Own Kudos [?]: 1237 [0]
Given Kudos: 0
Location: Orange County, CA
Send PM
Re: PS-Prep, time killer [#permalink] New post   28 Jun 2007, 12:55
Quote:

Also D. 1 minute.

1. Find pattern. There has to be one, otherwise it wouldn't be a GMAT question. Very quickly see that this thing is just alternating positive and negative powers of 1/2:
1/2, -1/4, 1/8, -1/16....

2. Every change is smaller than the previous one. Classic GMAT game. I drew a number line:

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2

Numbers start at 1/2, then down 1/4, then up 1/8, then down 1/16. You basically bounce back and fourth, but you NEVER cross 1/4, and you never cross 1/2.

-|---------------------|----------------------|
0 ....................... 1/4 ..................... 1/2
...................................................... Start
............................ {---------------------- ....(Back 1/4)
............................. ------------} ...............(forward 1/8)
......................................{----- ................(back 1/16)
.......................................---} .................(forward 1/32)
....................................... {- .................. (back 1/64)

Etc, etc. It's a lot here to type out, on paper, really simple. Either way, you can see the range you're in pretty clearly.


Thanks Ian. That's exactly what I was looking for. I figured there was some easier way. Now I just have to be able to find it and apply it on test day! :)
User avatar
andrehaui
Manager
Manager
Joined: 14 Mar 2007
Posts: 151
Own Kudos [?]: 22 [0]
Given Kudos: 0
Send PM
Re: PS-Prep, time killer [#permalink] New post   28 Jun 2007, 13:25
my aproach to this question:

first try 3 consecutive numbers

for k =1 : 0.5
for k=2 : -0.25
for k=3 : 0.125

so we have a geometric progression with ratio 1/-2....with this you can get your answer in less than 1minut



Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Problem Solving (PS) Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
GMAT Club Bot
gmatclubot
Re: PS-Prep, time killer [#permalink]
28 Jun 2007, 13:25
Moderators:
Bunuel
Math Expert
92875 posts
yashikaaggarwal
Senior Moderator - Masters Forum
3137 posts

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne
Main navigation
GMAT Resources
Partners
MBA Resources
www.gmac.com|www.mba.com | | GMAT Club Rules | Terms and Conditions