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

 It is currently 18 Aug 2018, 07:51

### 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

# Four hours from now, the population of a colony of bacteria will reach

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

### Hide Tags

Board of Directors
Joined: 01 Sep 2010
Posts: 3445
Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

02 Jul 2017, 12:00
4
Top Contributor
3
00:00

Difficulty:

55% (hard)

Question Stats:

64% (01:18) correct 36% (01:28) wrong based on 358 sessions

### HideShow timer Statistics

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 population 12 hours ago?

A. 6.4 * $$10 ^ 2$$
B. 8.0 * $$10 ^ 4$$
C. 1.6 * $$10 ^ 5$$
D. 3.2 * $$10 ^ 5$$
E. 8.0 * $$10 ^ 6$$

_________________
Intern
Joined: 26 Mar 2017
Posts: 18
Location: India
Schools: Great Lakes '19
GMAT 1: 580 Q47 V23
GPA: 3.1
Re: Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

02 Jul 2017, 12:16
2
Population after 4 hours is given, take it as x

So current population is x/2

Before 12 hours is x/16

Option B

Sent from my iPhone using GMAT Club Forum
BSchool Forum Moderator
Joined: 26 Feb 2016
Posts: 3042
Location: India
GPA: 3.12
Re: Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

02 Jul 2017, 12:18
These kind of questions, are best done when we go backwards.

1280000 when halved gives 640000(4 hours ago)
4 more hours ago the population of the colony of bacterial colony would be 320000
4 more hours ago, the population will be 160000
4 more hours ago, the population will be 80000 which is 8 * $$10 ^ 4$$(Option B)
_________________

You've got what it takes, but it will take everything you've got

CEO
Joined: 12 Sep 2015
Posts: 2705
Re: Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

02 Jul 2017, 14:48
3
Top Contributor
carcass 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 population 12 hours ago?

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

Let's work backwards.

Population 4 hours in future: 1.28 x 10^6
Population now: 0.64 x 10^6 (half the population 4 hours in future)
Population 4 hours ago: 0.32 x 10^6 (half the current population)
Population 8 hours ago: 0.16 x 10^6 (half the population 4 hours ago)
Population 12 hours ago: 0.08 x 10^6 (half the population 8 hours ago)

Now check the answer choices.
Only answer choices B and E have the same format with 8 times some power of 10
Answer choice E definitely doesn't match, so the correct answer must be B

For any doubters out there, notice that:
0.08 x 10^6 = (8.0) x (10^-2) x 10^6
= 8.0 x 10^4

RELATED VIDEO

_________________

Brent Hanneson – Founder of gmatprepnow.com

Intern
Joined: 31 Jul 2017
Posts: 8
Re: Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

16 Oct 2017, 18:23
It is possible to do using powers to simplify:
(future) 1.28 x 10^6
Going back 4 times from +4y to -12y: multiply by 1/(2^4)
1.28 x 10^6 = 128 x 10^4 = 2^7 x 10^4 == you could stop here looking to the possible answers
(2^7 x 10^4) / 2^4 = 2^3 x 10^4 = 8 x 10^4
Target Test Prep Representative
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 3180
Location: United States (CA)
Re: Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

19 Oct 2017, 10:26
carcass 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 population 12 hours ago?

A. 6.4 * $$10 ^ 2$$
B. 8.0 * $$10 ^ 4$$
C. 1.6 * $$10 ^ 5$$
D. 3.2 * $$10 ^ 5$$
E. 8.0 * $$10 ^ 6$$

Since the population doubles every 4 hours and the population will be 1.28 x 10^6 four hours from now, the current population must be half of 1.28 x 10^6, or ½(1.28 x 10^6) = 0.64 x 10^6.

Thus:

4 hours ago, the population was half the current population: ½(0.64 x 10^6) = 0.32 x 10^6.

8 hours ago, the population was half the population 4 hours ago: ½(0.32 x 10^6) = 0.16 x 10^6.

Finally, 12 hours ago the population was half the population 8 hours ago: ½(0.16 x 10^6) = 0.08 x 10^6 = 8 x 10^-2 x 10^6 = 8 x 10^4.

_________________

Scott Woodbury-Stewart
Founder and CEO

GMAT Quant Self-Study Course
500+ lessons 3000+ practice problems 800+ HD solutions

Manager
Joined: 17 Sep 2017
Posts: 61
Re: Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

15 Jan 2018, 03:33
It will double every 4 hour => if 4 hours from now the population is 1.28 * 10^6
=> now the population is 1.28 * 10^6 / 2

It will double (x2) every 4 hour => 12 hours ago, the population is 1/ 2^ 3 = 1/8 the population now (3 = 12/4)
=> The population 12 hours ago = 1.28 * 10^6 /2 * 1/8 = 8 * 10^4

=> B
Director
Joined: 09 Mar 2016
Posts: 763
Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

06 May 2018, 06:19
carcass 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 population 12 hours ago?

A. 6.4 * $$10 ^ 2$$
B. 8.0 * $$10 ^ 4$$
C. 1.6 * $$10 ^ 5$$
D. 3.2 * $$10 ^ 5$$
E. 8.0 * $$10 ^ 6$$

pushpitkc, is my approach / reasoning correct ?

1.28 * $$10 ^ 6$$ means we need to move decimal point 6 points to the right so it is 1,280,000, so i just leave out zeros and work with 128

if in 4 hours the population will be 128, then now it is 64

4 hours ago it was 32
8 hours ago 16
12 hrs ago 8

so all i need is to add 4 zeros and express it like this $$8*10^4$$

one thing I didn't get why in answer option B. 8.0 * $$10 ^ 4$$ $$8$$ is expressed as $$8.0$$ and not just $$8$$ I mean with one zero after decimal point...
_________________

In English I speak with a dictionary, and with people I am shy.

BSchool Forum Moderator
Joined: 26 Feb 2016
Posts: 3042
Location: India
GPA: 3.12
Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

06 May 2018, 08:13
1
dave13 wrote:
carcass 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 population 12 hours ago?

A. 6.4 * $$10 ^ 2$$
B. 8.0 * $$10 ^ 4$$
C. 1.6 * $$10 ^ 5$$
D. 3.2 * $$10 ^ 5$$
E. 8.0 * $$10 ^ 6$$

pushpitkc, is my approach / reasoning correct ?

1.28 * $$10 ^ 6$$ means we need to move decimal point 6 points to the right so it is 1,280,000, so i just leave out zeros and work with 128

if in 4 hours the population will be 128, then now it is 64

4 hours ago it was 32
8 hours ago 16
12 hrs ago 8

so all i need is to add 4 zeros and express it like this $$8*10^4$$

one thing I didn't get why in answer option B. 8.0 * $$10 ^ 4$$ $$8$$ is expressed as $$8.0$$ and not just $$8$$ I mean with one zero after decimal point...

Hi dave13

This problem is best solved when we go backwards!

8 *10^4 = 80000
Now see the sequence when we double the population of the colony of bacteria
80000 * 2 = 160000 | 160000 * 2 = 320000 | 320000 * 2 = 640000 | 640000 * 2 = 1280000
(You can see that 12 hours from the start, the population has increased
from 8 *10^4 to 6.4 *10^5 and then to 1.28 * 10^6)

Your approach is perfect otherwise. All you need to understand is this part of why the ZEROES increase!

Hope this helps you!
_________________

You've got what it takes, but it will take everything you've got

Manager
Joined: 29 Sep 2017
Posts: 113
Location: United States
GMAT 1: 720 Q49 V39
GPA: 3.3
WE: Consulting (Consulting)
Re: Four hours from now, the population of a colony of bacteria will reach  [#permalink]

### Show Tags

06 May 2018, 14:13
1.28E6 = 128E4. 12 hours before now = 2^4 --> 128E4 / 2^4 = 128E4 / 16 = 128/16 = 8E4.
_________________

If this helped, please give kudos!

Re: Four hours from now, the population of a colony of bacteria will reach &nbs [#permalink] 06 May 2018, 14:13
Display posts from previous: Sort by

# Four hours from now, the population of a colony of bacteria will reach

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

# Events & Promotions

 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®.