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# A certain bacteria colony doubles in size every day for 20

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Manager
Status: Retaking next month
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Location: India
Concentration: Marketing, Entrepreneurship
GMAT 1: 570 Q42 V27
GPA: 3.01
WE: Sales (Manufacturing)
A certain bacteria colony doubles in size every day for 20 [#permalink]

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30 Mar 2012, 02:35
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Difficulty:

55% (hard)

Question Stats:

58% (01:29) correct 42% (01:23) wrong based on 111 sessions

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A certain bacteria colony doubles in size every day for 20 days, a which point it reaches the limit of its habitat and can no longer grow. If two bacteria colonies start growing simultaneously, how many days will it take them to reach the habitat’s limit?

A. 6.33
B. 7.5
C. 10
D. 15
E. 19

OPEN DISCUSSION OF THIS QUESTION IS HERE: a-certain-bacteria-colony-doubles-in-size-every-day-for-144013.html
[Reveal] Spoiler: OA

Kudos [?]: 180 [0], given: 42

Intern
Joined: 30 Mar 2012
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Re: A certain bacteria colony doubles in size every day for 20 [#permalink]

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30 Mar 2012, 03:06
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Case 1: Consider the bacteria num to be B. As it doubles every day, number of bacteria at the end of each day would be
Day 1 – B
Day 2 – 2B
Day 3 – 4B
Day 4 – 8B
.
.
.
Day 20 – 2^19 B
It can be implied that the capacity of the habitat is 2^19 B.

Case 2: There are 2 bacteria which keep doubling every day. So, the count would be
Day 1 – 2B
Day 2 – 4B
Day 3 – 8B
.
.
.
Day 18 – 2^18 B
Day 19 – 2^19 B --> this is the point where the habitat is filled to its capacity.
Hence, 19 is the answer

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Senior Manager
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Re: A certain bacteria colony doubles in size every day for 20 [#permalink]

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30 Mar 2012, 04:20
I think the question is incorrect as number of days is not mentioned
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Re: A certain bacteria colony doubles in size every day for 20 [#permalink]

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14 Sep 2012, 00:55
simultaneous grow = same amount of time

In 20 days we should expect to have P(2)^20

So if we have two things doing the job of getting us there then;

2^x + 2^x = 20

2(2)^x=2
2^(x+1)=2^20

x+1 = 20

x = 19

We do not need additional information to answer this.
More often than not, if you start thinking the question is wrong chances are you are wrong!

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Intern
Joined: 09 Dec 2012
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Re: A certain bacteria colony doubles in size every day for 20 [#permalink]

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14 Dec 2012, 20:07
samsonfred76 wrote:
simultaneous grow = same amount of time

In 20 days we should expect to have P(2)^20

So if we have two things doing the job of getting us there then;

2^x + 2^x = 20

2(2)^x=2
2^(x+1)=2^20

x+1 = 20

x = 19

We do not need additional information to answer this.
More often than not, if you start thinking the question is wrong chances are you are wrong!

Please explain fully! I don't get it!

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Posts: 42257

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Re: A certain bacteria colony doubles in size every day for 20 [#permalink]

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15 Dec 2012, 05:09
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GMATCan wrote:
Please explain fully! I don't get it!

A certain bacteria colony doubles in size every day for 20 days, at which point it reaches the limit of its habitat and can no longer grow. If two bacteria colonies start growing simultaneously, how many days will it take them to reach the habitat’s limit?

A. 6.33
B. 7.5
C. 10
D. 15
E. 19

If there is one bacteria colony, then it will reach the limit of its habitat in 20 days.

If there are two bacteria colonies, then in order to reach the limit of habitat they would need to double one time less than in case with one colony. Thus colonies need to double 19 times.

Answer: E.

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Hope it helps.

OPEN DISCUSSION OF THIS QUESTION IS HERE: a-certain-bacteria-colony-doubles-in-size-every-day-for-144013.html
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Kudos [?]: 132702 [1], given: 12335

Re: A certain bacteria colony doubles in size every day for 20   [#permalink] 15 Dec 2012, 05:09
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# A certain bacteria colony doubles in size every day for 20

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