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

It is currently 20 Jul 2018, 19:20

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.

Close

Request Expert Reply

Confirm Cancel

The population of a bacteria culture doubles every 2 minutes

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Expert Post
6 KUDOS received
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 47157
The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 11 Feb 2014, 01:00
6
17
00:00
A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

69% (01:07) correct 31% (01:23) wrong based on 736 sessions

HideShow timer Statistics

The Official Guide For GMAT® Quantitative Review, 2ND Edition

The population of a bacteria culture doubles every 2 minutes. Approximately how many minutes will it take for the population to grow from 1,000 to 500,000 bacteria?

(A) 10
(B) 12
(C) 14
(D) 16
(E) 18

Problem Solving
Question: 90
Category: Arithmetic Estimation
Page: 73
Difficulty: 600


GMAT Club is introducing a new project: The Official Guide For GMAT® Quantitative Review, 2ND Edition - Quantitative Questions Project

Each week we'll be posting several questions from The Official Guide For GMAT® Quantitative Review, 2ND Edition and then after couple of days we'll provide Official Answer (OA) to them along with a slution.

We'll be glad if you participate in development of this project:
1. Please provide your solutions to the questions;
2. Please vote for the best solutions by pressing Kudos button;
3. Please vote for the questions themselves by pressing Kudos button;
4. Please share your views on difficulty level of the questions, so that we have most precise evaluation.

Thank you!

_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
Extra-hard Quant Tests with Brilliant Analytics

Most Helpful Expert Reply
Expert Post
4 KUDOS received
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 47157
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 11 Feb 2014, 01:01
4
11
SOLUTION

The population of a bacteria culture doubles every 2 minutes. Approximately how many minutes will it take for the population to grow from 1,000 to 500,000 bacteria?

(A) 10
(B) 12
(C) 14
(D) 16
(E) 18

\(1000 * 2^t = 500,000\) --> \(2^t = 500\) --> since \(2^9 = 512\), then \(t\approx{9}\). Thus, approximately 9 two-minute intervals are needed, whcih is 18 minutes.

Answer: E.

Similar questions to practice:
a-certain-bacteria-colony-doubles-in-size-every-day-for-144013.html
it-takes-30-days-to-fill-a-laboratory-dish-with-bacteria-140269.html
a-certain-culture-of-bacteria-quadruples-every-hour-if-a-52258.html
the-number-of-water-lilies-on-a-certain-lake-doubles-every-142858.html
the-population-of-locusts-in-a-certain-swarm-doubles-every-90353.html
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
Extra-hard Quant Tests with Brilliant Analytics

General Discussion
2 KUDOS received
Intern
Intern
avatar
Joined: 05 Feb 2014
Posts: 4
Location: Singapore
Concentration: Strategy, General Management
GMAT 1: 610 Q48 V27
GMAT 2: 730 Q49 V41
GPA: 2.8
WE: Sales (Energy and Utilities)
GMAT ToolKit User
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post Updated on: 11 Feb 2014, 01:47
2
Solution =

Let x be the total number of minutes. As the population doubles every 2 minutes, the number of times population doubles is \(y = \frac{x}{2}\)
Let Po be the Original Population of bacteria. = 1,000
Let Pn be the Final Population of bacteria. = 500,000

Therefore

\(Pn= Po*2^y\)
\(500000 = 1000 * 2^y\)
\(500 = 2^y\)
\((4*125) = 2^y\)

As 125~128

\((4*128) = 2^y\)
\((2^2*2^7) = 2^y\)
\((2^9) = 2^y\)
\(9 = y\)
\(\frac{x}{2} = 9\)
\(x = 18\)

Answer = E

Originally posted by ankitsachdev on 11 Feb 2014, 01:43.
Last edited by ankitsachdev on 11 Feb 2014, 01:47, edited 1 time in total.
4 KUDOS received
Intern
Intern
avatar
Joined: 30 Jan 2014
Posts: 18
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 11 Feb 2014, 01:44
4
The population of a bacteria culture doubles every 2 minutes. Approximately how many minutes will it take for the population to grow from 1,000 to 500,000 bacteria?

(A) 10
(B) 12
(C) 14
(D) 16
(E) 18

Lets say t is th number of times the population needs to grow
The question is \(1000*2^t=500000\)
Or \(2^t=500\)
2^6=64
2^7=128
2^8=256
2^9=512 BINGO.

So it needs to grow 9 times. If it grows every 2 min so \(9*2=18\)

Ans is E
_________________

Please +1 KUDO if my post helps. Thank you.

2 KUDOS received
Manager
Manager
avatar
Joined: 04 Jan 2014
Posts: 120
GMAT 1: 660 Q48 V32
GMAT 2: 630 Q48 V28
GMAT 3: 680 Q48 V35
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 11 Feb 2014, 01:48
2
We can ignore the thousands. So, basically, we are left with 1 and 500. Since, 2^9 = 512, approx minutes = 9*2 = 18. Answer (E).
Expert Post
1 KUDOS received
EMPOWERgmat Instructor
User avatar
V
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 12004
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 21 May 2015, 20:15
1
3
Hi All,

This prompt can be solved with a bit of "brute force" - you can essentially "count doubles" and find the answer. This method is probably faster than any other approach that you might take. You can even speed up by ignoring the last 3 zeroes.

1k
becomes
2k
4k
8k
16k
32k
64k
128k
256k
512k

9 "doubles" at 2 minutes each = about 18 minutes

Final Answer:

GMAT assassins aren't born, they're made,
Rich
_________________

760+: Learn What GMAT Assassins Do to Score at the Highest Levels
Contact Rich at: Rich.C@empowergmat.com

Rich Cohen

Co-Founder & GMAT Assassin

Special Offer: Save $75 + GMAT Club Tests Free
  Official GMAT Exam Packs + 70 Pt. Improvement Guarantee
www.empowergmat.com/

***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!***********************

Intern
Intern
avatar
B
Joined: 12 Aug 2013
Posts: 45
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 05 May 2016, 21:42
I am confused here slightly ...

If I try to solve the question GP way, I get slightly different answer :-

r = 2 ( as it doubles every 2 mins)
a = 1000
nth term = 500,000

ar^n-1 = 500,000
1000 * (2)^n-1 = 500,000
2^n-1 = 500
n-1 = 9
n = 10

and since in every 2 mins it gets doubled ..therefore 2* 10 = 20 mins.

Where am I going wrong?
Expert Post
EMPOWERgmat Instructor
User avatar
V
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 12004
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 05 May 2016, 23:07
Hi ravisinghal,

It looks like your work includes all of the terms (and it "counts" the 1st term, when that is technically the 'starting' term - so it should NOT be counted).

GMAT assassins aren't born, they're made,
Rich
_________________

760+: Learn What GMAT Assassins Do to Score at the Highest Levels
Contact Rich at: Rich.C@empowergmat.com

Rich Cohen

Co-Founder & GMAT Assassin

Special Offer: Save $75 + GMAT Club Tests Free
  Official GMAT Exam Packs + 70 Pt. Improvement Guarantee
www.empowergmat.com/

***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!***********************

Intern
Intern
avatar
B
Joined: 12 Aug 2013
Posts: 45
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 05 May 2016, 23:18
EMPOWERgmatRichC wrote:
Hi ravisinghal,

It looks like your work includes all of the terms (and it "counts" the 1st term, when that is technically the 'starting' term - so it should NOT be counted).

GMAT assassins aren't born, they're made,
Rich


can you please elaborate?

what I understand is 1000 should not be considered as 'a' ?? But that's the first term as per ques
Expert Post
EMPOWERgmat Instructor
User avatar
V
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 12004
Location: United States (CA)
GMAT 1: 800 Q51 V49
GRE 1: Q170 V170
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 06 May 2016, 11:53
Hi ravisinghal,

The question asks approximately how many minutes it takes for that increase to happen. If you START with 1,000 then no time has passed yet. If you're going to approach the question in the way that you did, then you can't include that first term since it requires NO time.

GMAT assassins aren't born, they're made,
Rich
_________________

760+: Learn What GMAT Assassins Do to Score at the Highest Levels
Contact Rich at: Rich.C@empowergmat.com

Rich Cohen

Co-Founder & GMAT Assassin

Special Offer: Save $75 + GMAT Club Tests Free
  Official GMAT Exam Packs + 70 Pt. Improvement Guarantee
www.empowergmat.com/

***********************Select EMPOWERgmat Courses now include ALL 6 Official GMAC CATs!***********************

1 KUDOS received
Manager
Manager
avatar
Joined: 13 Apr 2016
Posts: 60
Location: India
GMAT 1: 640 Q50 V27
GPA: 3
WE: Operations (Hospitality and Tourism)
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 06 May 2016, 12:59
1
[quote="Bunuel"]The Official Guide For GMAT® Quantitative Review, 2ND Edition

The population of a bacteria culture doubles every 2 minutes. Approximately how many minutes will it take for the population to grow from 1,000 to 500,000 bacteria?

(A) 10
(B) 12
(C) 14
(D) 16
(E) 18


2^9*1000=512000
answer 18 minutes.
solved in 20 secs :)
Intern
Intern
avatar
B
Joined: 12 Aug 2013
Posts: 45
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 10 May 2016, 23:22
ravisinghal wrote:
I am confused here slightly ...

If I try to solve the question GP way, I get slightly different answer :-

r = 2 ( as it doubles every 2 mins)
a = 1000
nth term = 500,000

ar^n-1 = 500,000
1000 * (2)^n-1 = 500,000
2^n-1 = 500
n-1 = 9
n = 10

and since in every 2 mins it gets doubled ..therefore 2* 10 = 20 mins.

Where am I going wrong?


Bunnuel,

I am still not happy with the explanation provided on why the above calculation is wrong? please help.

GP starts with 1000 and last term is 500,000.Therefore a has to be 1000 ..I don't understand why this should not be counted?
Intern
Intern
avatar
Joined: 10 May 2016
Posts: 9
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 11 May 2016, 01:46
ravisinghal wrote:
I am confused here slightly ...

If I try to solve the question GP way, I get slightly different answer :-

r = 2 ( as it doubles every 2 mins)
a = 1000
nth term = 500,000

ar^n-1 = 500,000
1000 * (2)^n-1 = 500,000
2^n-1 = 500
n-1 = 9
n = 10

and since in every 2 mins it gets doubled ..therefore 2* 10 = 20 mins.

Where am I going wrong?



Dear Ravi, u are right on your approach...
According to GP way 10th term would be 512000
So between 1st and 10th term there are 9 intervals of 2 min each means 18 Min

Ans : 18 Min E
Director
Director
User avatar
G
Joined: 09 Mar 2016
Posts: 656
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 25 Mar 2018, 08:21
Bunuel wrote:
SOLUTION

The population of a bacteria culture doubles every 2 minutes. Approximately how many minutes will it take for the population to grow from 1,000 to 500,000 bacteria?

(A) 10
(B) 12
(C) 14
(D) 16
(E) 18

\(1000 * 2^t = 500,000\) --> \(2^t = 500\) --> since \(2^9 = 512\), then \(t\approx{9}\). Thus, approximately 9 two-minute intervals are needed, whcih is 18 minutes.

Answer: E.

Similar questions to practice:
http://gmatclub.com/forum/a-certain-bac ... 44013.html
http://gmatclub.com/forum/it-takes-30-d ... 40269.html
http://gmatclub.com/forum/a-certain-cul ... 52258.html
http://gmatclub.com/forum/the-number-of ... 42858.html
http://gmatclub.com/forum/the-populatio ... 90353.html



how should i know that 2^9 is 512... what if i have no idea about it :?

and what does 500 mean :?
1 KUDOS received
Manager
Manager
User avatar
G
Joined: 16 Sep 2016
Posts: 225
WE: Analyst (Health Care)
Premium Member CAT Tests
Re: The population of a bacteria culture doubles every 2 minutes  [#permalink]

Show Tags

New post 25 Mar 2018, 08:54
1
dave13 wrote:
Bunuel wrote:
SOLUTION

The population of a bacteria culture doubles every 2 minutes. Approximately how many minutes will it take for the population to grow from 1,000 to 500,000 bacteria?

(A) 10
(B) 12
(C) 14
(D) 16
(E) 18

\(1000 * 2^t = 500,000\) --> \(2^t = 500\) --> since \(2^9 = 512\), then \(t\approx{9}\). Thus, approximately 9 two-minute intervals are needed, whcih is 18 minutes.

Answer: E.

Similar questions to practice:
http://gmatclub.com/forum/a-certain-bac ... 44013.html
http://gmatclub.com/forum/it-takes-30-d ... 40269.html
http://gmatclub.com/forum/a-certain-cul ... 52258.html
http://gmatclub.com/forum/the-number-of ... 42858.html
http://gmatclub.com/forum/the-populatio ... 90353.html



how should i know that 2^9 is 512... what if i have no idea about it :?

and what does 500 mean :?


Hey dave13, :-)

remembering certain powers of 2 can be very helpful ( and easy !)

You can start with 2^10 = 1024 ( or 1 MB - from data storage devices.. as all data storage devices have capacity in powers of 2 *fun fact* )

From here it is easy to get to other powers as we can always round off to nearest number of 1000.

Such numbers almost always arise in geometric series ( which this Q is an example of ).

Best,
Gladi
Re: The population of a bacteria culture doubles every 2 minutes &nbs [#permalink] 25 Mar 2018, 08:54
Display posts from previous: Sort by

The population of a bacteria culture doubles every 2 minutes

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  

Events & Promotions

PREV
NEXT


GMAT Club MBA Forum Home| About| Terms and Conditions and Privacy Policy| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne

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®.