GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 20 Jun 2018, 23:31

### 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

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

# If 2/3 of the air in a tank is removed with each stroke of a

Author Message
TAGS:

### Hide Tags

Director
Status: Finally Done. Admitted in Kellogg for 2015 intake
Joined: 25 Jun 2011
Posts: 514
Location: United Kingdom
GMAT 1: 730 Q49 V45
GPA: 2.9
WE: Information Technology (Consulting)
If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

01 Mar 2012, 16:58
2
9
00:00

Difficulty:

65% (hard)

Question Stats:

53% (01:17) correct 47% (01:27) wrong based on 452 sessions

### HideShow timer Statistics

If 2/3 of the air in a tank is removed with each stroke of a vacuum pump, how many strokes does it take before less than 1% of the original amount of air in the tank remains?

(A) 2
(B) 3
(C) 4
(D) 5
(E) 6

What's the best way to solve this question guys? By picking a number say or algebraically?

_________________

Best Regards,
E.

MGMAT 1 --> 530
MGMAT 2--> 640
MGMAT 3 ---> 610
GMAT ==> 730

Math Expert
Joined: 02 Sep 2009
Posts: 46217
Re: If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

01 Mar 2012, 17:18
9
7
enigma123 wrote:
If 2/3 of the air in a tank is removed with each stroke of a vacuum pump, how many strokes does it take before less than 1% of the original amount of air in the tank remains?

(A) 2
(B) 3
(C) 4
(D) 5
(E) 6

What's the best way to solve this question guys? By picking a number say or algebraically?

2/3 of the air in a tank is removed with each stroke means that 1/3 of the air is left after one stroke, 1/3*1/3 after two strokes and so on. Basically after n strokes there will be (1/3)^n of the air left in the tank. The question asks about such n for which (1/3)^n<1/100 --> 3^n>100 --> n=5.

Similar question to practice: if-3-4-of-the-mineral-deposits-in-a-reservoir-of-water-are-97300.html

Hope it helps.
_________________
Manager
Joined: 28 Feb 2012
Posts: 112
GPA: 3.9
WE: Marketing (Other)
Re: If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

29 May 2013, 23:09
2
I love the solutions provided by Bunuel, but not always can come up with such logic, which is simple and straightforward, but i am trying to.
My approach is to find more or less good number and plug in. Lets take 90 as the total volume of an air, and the question asks after how many stokes there will be less than 0,9 air in a tank, if each stroke takes 2/3 of an air.
So after he 1 stroke the volume will be 30 (90-60=30), and so on
2 stroke - 30-20=10
3 stroke - 10-6,7=3,3
4 stroke - 3,3-2,2=1,1
5 stroke - 1,1 - minus something that gets us less than 0,9 for sure. so the answer is: after 5 strokes there will be less than 1% of air left in a tank. Option D.
_________________

If you found my post useful and/or interesting - you are welcome to give kudos!

SVP
Joined: 06 Sep 2013
Posts: 1883
Concentration: Finance
Re: If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

03 Jan 2014, 08:08
enigma123 wrote:
If 2/3 of the air in a tank is removed with each stroke of a vacuum pump, how many strokes does it take before less than 1% of the original amount of air in the tank remains?

(A) 2
(B) 3
(C) 4
(D) 5
(E) 6

What's the best way to solve this question guys? By picking a number say or algebraically?

Well yeah algebraically it would be (1/3)^x <1/100

So x>=5

There you go, answer is D

Cheers!
If it helps consider providing some Kudos ok?
J
Director
Status: Everyone is a leader. Just stop listening to others.
Joined: 22 Mar 2013
Posts: 893
Location: India
GPA: 3.51
WE: Information Technology (Computer Software)
Re: If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

29 Mar 2014, 05:50
This question is one of the standard-type.

Whenever something increases or decreases by some percent on each cycle, your mind should start thinking about application of compound interest formula.

$$Final-amount = Original-amount [ 1 - \frac{Rate}{100}]^n --- for -decreasing- rate.$$
$$Final-amount = Original-amount [ 1 + \frac{Rate}{100}]^n --- for- increasing- rate.$$

For current question solve following equation:
$$1 > 100 [ 1 - \frac{\frac{200}{3}}{100}]^n$$

$$1 > 100 [ 1 - \frac{200}{300}]^n$$

for n = 5,6... this relation exist.

_________________

Piyush K
-----------------------
Our greatest weakness lies in giving up. The most certain way to succeed is to try just one more time. ― Thomas A. Edison
Don't forget to press--> Kudos
My Articles: 1. WOULD: when to use? | 2. All GMATPrep RCs (New)
Tip: Before exam a week earlier don't forget to exhaust all gmatprep problems specially for "sentence correction".

Manager
Joined: 21 Jul 2017
Posts: 113
Location: India
GMAT 1: 650 Q47 V33
GPA: 4
Re: If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

20 Aug 2017, 04:34
2/3 of the air in a tank is removed with each stroke, that's 1/3 of the air is left after one stroke, 1/3*1/3 after two strokes and so on.
After x strokes there will be (1/3)^x of the air left in the tank. The question asks about such x for which (1/3)^x<1/100 --> 3^x>100 --> x=5
Senior Manager
Joined: 04 Oct 2015
Posts: 349
Location: Viet Nam
Concentration: Finance, Economics
GMAT 1: 730 Q51 V36
GPA: 3.56
Re: If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

20 Aug 2017, 17:44
Hi Bunuel !

I wonder if the answer is 4 because the stem clearly state does it take BEFORE less than 1%.

Please shed some light! Many thanks
_________________

Do not pray for an easy life, pray for the strength to endure a difficult one - Bruce Lee

Math Expert
Joined: 02 Sep 2009
Posts: 46217
Re: If 2/3 of the air in a tank is removed with each stroke of a [#permalink]

### Show Tags

21 Aug 2017, 02:26
leanhdung wrote:
Hi Bunuel !

I wonder if the answer is 4 because the stem clearly state does it take BEFORE less than 1%.

Please shed some light! Many thanks

I think it's clear that the question asks about the number of strokes that will make amount of the air less than 1%.
_________________
Re: If 2/3 of the air in a tank is removed with each stroke of a   [#permalink] 21 Aug 2017, 02:26
Display posts from previous: Sort by