It is currently 16 Dec 2017, 05:04

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

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

Four hours from now, the population of a colony of bacteria

  post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
Director
Director
avatar
Joined: 10 Feb 2006
Posts: 655

Kudos [?]: 641 [0], given: 0

Four hours from now, the population of a colony of bacteria [#permalink]

Show Tags

New post 04 May 2008, 07:06
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Four hours from now, the population of a colony of bacteria will reach
1.28 * 10 ^ 6. If the population of the colony doubles every 4 hours, what was the popluation 12 hours ago?

6.4 * 10 ^ 2
8.0 * 10 ^ 4
1.6 * 10 ^ 5
3.2 * 10 ^ 5
8.0 * 10 ^ 6

Here's how i did it. Not getting the answer please help :

NOW = x
4 hours from now = 2x = 1.28 * 10 ^ 6
x = (1.28 * 2 ^ 6 * 5 ^ 6)/2
x = 1.28 * 2 ^ 5 * 5 ^ 6

x/16 = 1.28 * 2 ^ 5 * 5 ^ 6 / 2 ^ 4 = 1.28 * 2 * 5 ^ 6

128/100 * 2 * 5 ^ 6
128 /50 * 5 ^ 6
(64/25) * 5 ^ 6
(64 /5 ^ 2) * 5 ^ 6
64 * 5 ^ 4
_________________

GMAT the final frontie!!!.

Kudos [?]: 641 [0], given: 0

Manager
Manager
avatar
Joined: 31 Oct 2007
Posts: 112

Kudos [?]: 74 [0], given: 0

Location: Frankfurt, Germany
Re: Bacteria [#permalink]

Show Tags

New post 04 May 2008, 07:15
1
This post was
BOOKMARKED
alimad wrote:
Four hours from now, the population of a colony of bacteria will reach
1.28 * 10 ^ 6. If the population of the colony doubles every 4 hours, what was the popluation 12 hours ago?

6.4 * 10 ^ 2
8.0 * 10 ^ 4
1.6 * 10 ^ 5
3.2 * 10 ^ 5
8.0 * 10 ^ 6


4hrs later - 1.28 * 10^6
Now - 0.64 * 10^6
4hrs ago - 0.32 * 10^6
8hrs ago - 0.16 * 10^6
12hrs ago - 0.08 * 10^6

0.08 * 10^6 = 8/10^2 * 10^6
= 8.0 * 10^4

Ans: B
_________________

Jimmy Low, Frankfurt, Germany
Blog: http://mytrainmaster.wordpress.com
GMAT Malaysia: http://gmatmalaysia.blogspot.com

Kudos [?]: 74 [0], given: 0

Intern
Intern
avatar
Joined: 21 Mar 2008
Posts: 22

Kudos [?]: [0], given: 0

Re: Bacteria [#permalink]

Show Tags

New post 04 May 2008, 07:20
alimad wrote:
Four hours from now, the population of a colony of bacteria will reach
1.28 * 10 ^ 6. If the population of the colony doubles every 4 hours, what was the popluation 12 hours ago?

6.4 * 10 ^ 2
8.0 * 10 ^ 4
1.6 * 10 ^ 5
3.2 * 10 ^ 5
8.0 * 10 ^ 6

Here's how i did it. Not getting the answer please help :

NOW = x
4 hours from now = 2x = 1.28 * 10 ^ 6
x = (1.28 * 2 ^ 6 * 5 ^ 6)/2
x = 1.28 * 2 ^ 5 * 5 ^ 6

x/16 = 1.28 * 2 ^ 5 * 5 ^ 6 / 2 ^ 4 = 1.28 * 2 * 5 ^ 6

128/100 * 2 * 5 ^ 6
128 /50 * 5 ^ 6
(64/25) * 5 ^ 6
(64 /5 ^ 2) * 5 ^ 6
64 * 5 ^ 4


You need to divide the number given by 16. Why 16? Well, you account (2x) for the number itself, and then, 12 hours ago, 12/4 = 3. 2^3 = 8, multiply by the 2 from step one and it's 16.

1.28 / 16 move two decimals over so 128 / 16 = 8 and then it's now 10^4 not 10^6 because you moved those decimals over.

Kudos [?]: [0], given: 0

Current Student
avatar
Joined: 28 Dec 2004
Posts: 3345

Kudos [?]: 325 [0], given: 2

Location: New York City
Schools: Wharton'11 HBS'12
Re: Bacteria [#permalink]

Show Tags

New post 04 May 2008, 07:51
the question says 4 hours from now..the population is 1.28*10^6...

doubles every 4 hrs..so 12 hours ago..what was the population..well basically we need to go back 16 hours from the 1.28*10^6...

16/4=4...basically 2^4.. 128*10^4/2^4=2^3*10^4 or 8*10^4

Kudos [?]: 325 [0], given: 2

Non-Human User
User avatar
Joined: 09 Sep 2013
Posts: 14817

Kudos [?]: 288 [0], given: 0

Premium Member
Re: Four hours from now, the population of a colony of bacteria [#permalink]

Show Tags

New post 19 Nov 2017, 01:30
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

GMAT Books | GMAT Club Tests | Best Prices on GMAT Courses | GMAT Mobile App | Math Resources | Verbal Resources

Kudos [?]: 288 [0], given: 0

Re: Four hours from now, the population of a colony of bacteria   [#permalink] 19 Nov 2017, 01:30
Display posts from previous: Sort by

Four hours from now, the population of a colony of bacteria

  post reply Question banks Downloads My Bookmarks Reviews Important topics  

Moderator: chetan2u



GMAT Club MBA Forum Home| About| Terms and Conditions| 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®.