Live GMAT classes with Target Test Prep start Nov 21 and Nov 22. First class free.

See classes
Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

Newsweek Readers’ Choice Awards 2026 winner: Target Test Prep, #1 Test Prep

Target Test Prep was voted #1 Best Test Prep in the 2026 Newsweek Readers’ Choice™ Awards.

★★★★★5.0563 reviews
GMATLiveTeachStarts Nov 22
Josh Braslow, Target Test Prep GMAT expert
LIVE ONLINE CLASSES

Get Ready for GMAT Test Day Faster with Live Online Classes

with Josh Braslow, 20+ years in test prep

Nov 22 · Josh Braslow
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes + 6 months of TTP OnDemand
  • Attend the first class for free
  • Every class is recorded, so you never fall behind
View classes & enroll
Limited seats availableTarget Test Prep
EALiveTeachOnDemandStart anytime
EXECUTIVE ASSESSMENT

Target Test Prep EA OnDemand

Self-paced EA prep. Study on your schedule.

EXECUTIVE ASSESSMENT

with

165+ EA score guarantee
$05-day trial no automatic billing
Schedule
Included
40 hours of live online classes plus six months of access to the complete TTP EA OnDemand course.
  • 165+ EA Score Guarantee
  • 4,100+ Quant, Verbal, and Integrated Reasoning practice questions
  • 400+ hours of in-depth video lessons
  • 3,000+ step-by-step video solutions
View EA class & enroll Start free 5-day trial
Limited cohort · enrollment openTrial includes full course accessTarget Test Prep
GMATOnDemandStart anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

715+ score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions
Start free 5-day trial
Read 563 reviewsTarget Test Prep

Arithmetic

Expert replies
Post new topic Post Reply
Subscribe topic
Previous Topic Next Topic
User avatar
Master | Next Rank: 500 Posts
Posts: 123
Joined: Tue Nov 23, 2010 7:18 pmFollowed by: 4 members
Location: Mumbai, India
Thanked: 5 times
by anirudhbhalotia » Thu Jan 13, 2011 10:14 pm
If 1/2 of the air in a tank is removed with each stroke of a vacuum pump, what fraction of the original amount of air has been removed after 4 strokes?

A. 15/16

B. 7/8

C. 1/4

D. 1/8

E. 1/16

This question is from OG-12, the answer mentioned is A, but am unable to understand the explanation. I had chosen E as the answer.
Top
Join the discussion
Source: — Problem Solving |

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 3835
Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members
Location: Milpitas, CA
Thanked: 1854 times
GMAT Score:770

by Anurag@Gurome » Thu Jan 13, 2011 10:57 pm
anirudhbhalotia wrote:If 1/2 of the air in a tank is removed with each stroke of a vacuum pump, what fraction of the original amount of air has been removed after 4 strokes?

A. 15/16
B. 7/8
C. 1/4
D. 1/8
E. 1/16
In each stroke it removes (1/2) of the air in the tank. Therefore after each stroke the amount of remaining air in the tank is (1/2) of what was before the stroke. Thus,
  • After 1st stroke : Remaining air = (1/2) of original amount
    After 2nd stroke : Remaining air = (1/2)*(1/2) of original amount
    After 3rd stroke : Remaining air = (1/2)*(1/2)*(1/2) of original amount
    After 4th stroke : Remaining air = (1/2)*(1/2)*(1/2)*(1/2) of original amount
Hence after four strokes remaining air in the tank = (1/16) of the original amount

Hence the amount of air removed = [1 - (1/16)] of the original amount = (15/16) of the original amount

The correct answer is A.[/list]
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)

Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
Top
Join the discussion
Master | Next Rank: 500 Posts
Posts: 155
Joined: Mon Dec 13, 2010 11:02 am
Thanked: 3 times

by towerSpider » Thu Jan 13, 2011 10:59 pm
anirudhbhalotia wrote:If 1/2 of the air in a tank is removed with each stroke of a vacuum pump, what fraction of the original amount of air has been removed after 4 strokes?

A. 15/16

B. 7/8

C. 1/4

D. 1/8

E. 1/16

This question is from OG-12, the answer mentioned is A, but am unable to understand the explanation. I had chosen E as the answer.
(1/2)^4=1/16 This is amount left, so amount removed: 15/16
Top
Join the discussion
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members
Location: New York, NY
Thanked: 13060 times
GMAT Score:790

by GMATGuruNY » Fri Jan 14, 2011 4:34 am
anirudhbhalotia wrote:If 1/2 of the air in a tank is removed with each stroke of a vacuum pump, what fraction of the original amount of air has been removed after 4 strokes?

A. 15/16

B. 7/8

C. 1/4

D. 1/8

E. 1/16

This question is from OG-12, the answer mentioned is A, but am unable to understand the explanation. I had chosen E as the answer.
We can plug in a value for the tank. Since each stroke of the pump removes 1/2 the tank, we should plug in a value that can divided by 2 four times

Let tank = 16.
Stroke 1 removes 1/2*16 = 8, so 16-8 = 8 left.
Stroke 2 removes 1/2*8 = 4, so 8-4 = 4 left.
Stroke 3 removes 1/2*4 = 2, so 4-2 = 2 left.
Stroke 4 removes 1/2*2 = 1, so 2-1 = 1 left.
Since 1 is left, 16-1 = 15 was removed.
Amount removed/Tank = 15/16.

The correct answer is A.
Last edited by GMATGuruNY on Sun Jan 16, 2011 3:14 am, edited 1 time in total.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
Join the discussion
User avatar
Master | Next Rank: 500 Posts
Posts: 123
Joined: Tue Nov 23, 2010 7:18 pmFollowed by: 4 members
Location: Mumbai, India
Thanked: 5 times

by anirudhbhalotia » Sun Jan 16, 2011 2:22 am
I am not understanding this one...if 1/16 is removed...thats the answer right?

Why are we subtracting from 1...if we subtract that much remains...is it not?
Top
Join the discussion
User avatar
Master | Next Rank: 500 Posts
Posts: 261
Joined: Wed Mar 31, 2010 8:37 pmFollowed by: 3 members
Location: Varanasi
Thanked: 11 times

by ankur.agrawal » Sun Jan 16, 2011 2:35 am
anirudhbhalotia wrote:I am not understanding this one...if 1/16 is removed...thats the answer right?

Why are we subtracting from 1...if we subtract that much remains...is it not?
Ur Point makes sense. lets analyze it this way:

Let tank = 16.
Stroke 1 removes 1/2*16 = 8
Stroke 2 removes 1/2*8 = 4
Stroke 3 removes 1/2*4 = 2
Stroke 4 removes 1/2*2 = 1

So Total Removed=8+4+2+1 = 15

So the fraction of the original amount of air has been removed after 4 strokes= 15/16.

Hope this helps.
Top
Join the discussion
User avatar
Master | Next Rank: 500 Posts
Posts: 123
Joined: Tue Nov 23, 2010 7:18 pmFollowed by: 4 members
Location: Mumbai, India
Thanked: 5 times

by anirudhbhalotia » Sun Jan 16, 2011 2:47 am
ankur.agrawal wrote:
anirudhbhalotia wrote:I am not understanding this one...if 1/16 is removed...thats the answer right?

Why are we subtracting from 1...if we subtract that much remains...is it not?
Ur Point makes sense. lets analyze it this way:

Let tank = 16.
Stroke 1 removes 1/2*16 = 8
Stroke 2 removes 1/2*8 = 4
Stroke 3 removes 1/2*4 = 2
Stroke 4 removes 1/2*2 = 1

So Total Removed=8+4+2+1 = 15

So the fraction of the original amount of air has been removed after 4 strokes= 15/16.

Hope this helps.
I think I got it now...I was looking at it a different way...your adding up somewhat got it cleared for me.

Thanks!
Top
Join the discussion