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# M06-18

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Math Expert
Joined: 02 Sep 2009
Posts: 43896

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15 Sep 2014, 23:27
Expert's post
10
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Difficulty:

55% (hard)

Question Stats:

53% (00:49) correct 47% (01:01) wrong based on 179 sessions

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The population of a bacteria colony doubles every day. If it was started 9 days ago with 2 bacteria and each bacteria lives for 12 days, how large is the colony today?

A. 512
B. 768
C. 1024
D. 2048
E. 4096
[Reveal] Spoiler: OA

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Math Expert
Joined: 02 Sep 2009
Posts: 43896

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15 Sep 2014, 23:27
1
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Expert's post
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Official Solution:

The population of a bacteria colony doubles every day. If it was started 9 days ago with 2 bacteria and each bacteria lives for 12 days, how large is the colony today?

A. 512
B. 768
C. 1024
D. 2048
E. 4096

We know that the population doubles every day; other information is not really relevant. If the population started ONE day ago (that is yesterday), it would be $$2^2$$ today. If the population started $$N$$ days ago, then today it is $$2^{(N+1)}$$. Today the population size is $$2^{10}$$, which is 1024.

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GMAT 1: 720 Q50 V38
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23 Oct 2014, 04:00
Bunuel wrote:
The population of a bacteria colony doubles every day. If it was started 9 days ago with 2 bacteria and each bacteria lives for 12 days, how large is the colony today?

A. 512
B. 768
C. 1024
D. 2048
E. 4096

This question was a sitter, still i got this wrong not because I didn't know the concepts well, but because I did a silly mistake. Since the information provided is of 9 days ago, I calculated the answer for 9 days itself. The calculation should be done for n+1 days.

It started with 2 bacteria, thus today, the number of bacteria will be $$2^10$$ $$=1024$$

The other information about the lives of bacteria is redundant, this information would have been useful, had the question asked for more than 11 days.

Ans - 1024
Choice C
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Intern
Joined: 11 Nov 2014
Posts: 39
Concentration: Technology, Strategy
GMAT 1: 660 Q48 V31
GMAT 2: 720 Q50 V37
GPA: 3.6
WE: Consulting (Consulting)

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29 Jul 2015, 05:45
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"Started 9 days ago" one can understand that we are on day 9....
Intern
Joined: 04 Sep 2015
Posts: 14
WE: Asset Management (Investment Banking)

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08 Nov 2015, 19:26
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The tricky part is "it was started 9 days ago" which means they are on day 10.
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Joined: 20 Sep 2015
Posts: 2
Schools: CBS '18, Stern '18

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09 Nov 2015, 22:58
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I got the correct answer, but I did it differently. I did not use the N+1 concept as I believe this could easily mean we are on the 9th day.

Let the population be : p
Rate of population expansion : 2^(N)

Therefore, the population after N days = p*2^(N).

In the question it is given p=2 and N =9. We plug it in and get 2*2^(9) = 2^(10) = 1024.

the logic of 2^(N+1) will not work if we started with a population of 100 and the population doubled everyday.

Let me know if I missed something here.
Intern
Joined: 16 Nov 2015
Posts: 5

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20 Mar 2016, 17:55
I think this is a poor-quality question and the explanation isn't clear enough, please elaborate. I understand the solution but I am slightly confused about the meaning of " 9 days ago ", In my opinion the wording of the question is ambiguous on when the bacteria colony doubles. Weather it is at the beginning, end or during the course of the day. I was wondering if there was a rule with this kind of wording on the GMAT. Please clarify if possible, Thank you.
Manager
Joined: 27 Feb 2015
Posts: 59
Concentration: General Management, Economics
GMAT 1: 630 Q42 V34
WE: Engineering (Transportation)

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10 Oct 2016, 13:02
Bunuel
Hi Bunuel, first of all thanks for the prompt replies.
Coming to my query, although the question was a sitter one, I spent lot of time thinking why the data about 12 days was given.
I have never come across a question where any of the data provided was not useful. Infact, I always think that If I cannot use any one of the data then I am missing the point. Does this happen on GMAT?
Manager
Joined: 03 Jun 2015
Posts: 86

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19 Apr 2017, 01:46
I solved the problem in this way-
TIME ELAPSED | BACTERIA
9 days ago ------ 2
8 days ago ------ 4
7 days ago ------ 8
6 days ago ------ 16
5 days ago ------ 32
4 days ago ------ 64
3 days ago ------ 128
2 days ago ------ 256
1 day ago ------ 512
NOW ------ 1024
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M06-18   [#permalink] 19 Apr 2017, 01:46
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# M06-18

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