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The population of City X doubles every 25 years. If this rate continue

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The population of City X doubles every 25 years. If this rate continue  [#permalink]

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New post 06 Apr 2017, 00:42
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The population of City X doubles every 25 years. If this rate continues for the foreseeable future, how many centuries will it take for the population to increase from 20,000 to 1,280,000?

A. 6
B. 4.5
C. 2.5
D. 2
E. 1.5

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Re: The population of City X doubles every 25 years. If this rate continue  [#permalink]

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New post 06 Apr 2017, 02:20
1
Bunuel wrote:
The population of City X doubles every 25 years. If this rate continues for the foreseeable future, how many centuries will it take for the population to increase from 20,000 to 1,280,000?

A. 6
B. 4.5
C. 2.5
D. 2
E. 1.5


20000---after 25 years---40000---after 25 years---80000---after 25 years---160000---after 25 years---320000---after 25 years---640000---after 25 years---1280000

so from 20000 to 1280000 it takes 25*6 years i.e. 150 years = 1.5 centuries

Hence option E is correct
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Re: The population of City X doubles every 25 years. If this rate continue  [#permalink]

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New post 06 Apr 2017, 06:01
1,280,000/20,000 = 64 times = 2^6 times

25*6 = 150 years = 1.5 centuries

Answer: E
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Re: The population of City X doubles every 25 years. If this rate continue  [#permalink]

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New post 06 Apr 2017, 07:14
answer is E


20000*2^n=1280000

2^n=64
n=6

it takes 25 years to double
6 times will take 25*6=150/100=1.5 centuries
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Re: The population of City X doubles every 25 years. If this rate continue  [#permalink]

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New post 12 Apr 2017, 11:05
1
Bunuel wrote:
The population of City X doubles every 25 years. If this rate continues for the foreseeable future, how many centuries will it take for the population to increase from 20,000 to 1,280,000?

A. 6
B. 4.5
C. 2.5
D. 2
E. 1.5



We are given that the population of City X doubles every 25 years. We need to determine the number of centuries it will take the population to increase from 20,000 to 1,280,000.

We can start by making a list (or table) that keeps track of doubling the population every 25 years:

Current: 20,000
After 25 years: 40,000
After 50 years: 80,000
After 75 years: 160,000
After 100 years: 320,000
After 125 years: 640,000
After 150 years: 1,280,000

Thus it takes 150 years, or 1.5 centuries, to increase the population from 20,000 to 1,280,000.

Answer: E
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Re: The population of City X doubles every 25 years. If this rate continue  [#permalink]

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New post 02 Dec 2018, 19:37
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).

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Re: The population of City X doubles every 25 years. If this rate continue &nbs [#permalink] 02 Dec 2018, 19:37
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