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# Population of a bacteria colony doubles every day. If it was

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CEO
Joined: 21 Jan 2007
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Population of a bacteria colony doubles every day. If it was [#permalink]

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12 Nov 2007, 12:31
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Population of a bacteria colony doubles every day. If it was started 6 days ago when 3 bacteria were born in a deserted colony and each bacteria lives for 12 days, how large is the colony today?

Kudos [?]: 1085 [0], given: 4

CEO
Joined: 21 Jan 2007
Posts: 2734

Kudos [?]: 1085 [0], given: 4

Location: New York City

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12 Nov 2007, 12:37
Ravshonbek wrote:
bmwhype2 wrote:
Population of a bacteria colony doubles every day. If it was started 6 days ago when 3 bacteria were born in a deserted colony and each bacteria lives for 12 days, how large is the colony today?

3*2^6=64*3=192

Can you explain the numbers?

A. What if it triples instead of doubles?
B. What if it doubles and started 4 days ago?
C. What if it triples and started 3 days ago?

Kudos [?]: 1085 [0], given: 4

VP
Joined: 09 Jul 2007
Posts: 1098

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12 Nov 2007, 12:43
bmwhype2 wrote:
Ravshonbek wrote:
bmwhype2 wrote:
Population of a bacteria colony doubles every day. If it was started 6 days ago when 3 bacteria were born in a deserted colony and each bacteria lives for 12 days, how large is the colony today?

3*2^6=64*3=192

Can you explain the numbers?

A. What if it triples instead of doubles?
B. What if it doubles and started 4 days ago?
C. What if it triples and started 3 days ago?

as you know
Double=multiplying by 2
Tripple=multiplying by 3

A. ((((3)*3)*3)....)*3^n = 3*3^6; n=6-number of days
B. 3*2^4=3*16
C. 3*3^3=3*27

Kudos [?]: 149 [0], given: 0

CEO
Joined: 21 Jan 2007
Posts: 2734

Kudos [?]: 1085 [0], given: 4

Location: New York City

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16 Nov 2007, 02:03
Ravshonbek wrote:
bmwhype2 wrote:
Population of a bacteria colony doubles every day. If it was started 6 days ago when 3 bacteria were born in a deserted colony and each bacteria lives for 12 days, how large is the colony today?

3*2^6=64*3=192

why do u multiply it again by 3?

Kudos [?]: 1085 [0], given: 4

Manager
Joined: 20 Sep 2006
Posts: 58

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16 Nov 2007, 04:27
Ravshonbek wrote:
bmwhype2 wrote:
Population of a bacteria colony doubles every day. If it was started 6 days ago when 3 bacteria were born in a deserted colony and each bacteria lives for 12 days, how large is the colony today?

3*2^6=64*3=192

Shouldnt the formula for this case be 3 x 2^n-1
where n=6

First day = 3
Second day = 3x2^1
Third day = 3x2^2
.
.
Sixth day = 3x2^5 = 96

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Intern
Joined: 14 Nov 2007
Posts: 10

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16 Nov 2007, 04:51
saviop wrote:
Ravshonbek wrote:
bmwhype2 wrote:
Population of a bacteria colony doubles every day. If it was started 6 days ago when 3 bacteria were born in a deserted colony and each bacteria lives for 12 days, how large is the colony today?

3*2^6=64*3=192

Shouldnt the formula for this case be 3 x 2^n-1
where n=6

First day = 3
Second day = 3x2^1
Third day = 3x2^2
.
.
Sixth day = 3x2^5 = 96

I agree with this. 192 will be on the 7th day. The answer is 96

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Director
Joined: 26 Jul 2007
Posts: 535

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Schools: Stern, McCombs, Marshall, Wharton

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12 Dec 2007, 09:47
bmwhype2 wrote:
OA is 96

I dont see how the answer is 96.

If 6 days ago there were 3, then
5 days ago there were 6, then
4 days ago there were 12, then
3 days ago there were 24, then
2 days ago there were 48, then
1 day ago there were 96, then
Today there are 192.

Six days ago means today is the 7th day.

Kudos [?]: 192 [0], given: 0

CEO
Joined: 21 Jan 2007
Posts: 2734

Kudos [?]: 1085 [0], given: 4

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12 Dec 2007, 10:25
gixxer1000 wrote:
bmwhype2 wrote:
OA is 96

I dont see how the answer is 96.

If 6 days ago there were 3, then
5 days ago there were 6, then
4 days ago there were 12, then
3 days ago there were 24, then
2 days ago there were 48, then
1 day ago there were 96, then
Today there are 192.

Six days ago means today is the 7th day.

sorry. you are correct. i was looking at the variation of this question.

Kudos [?]: 1085 [0], given: 4

12 Dec 2007, 10:25
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# Population of a bacteria colony doubles every day. If it was

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