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m12#11 - Number System

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m12#11 - Number System [#permalink] New post 12 Jun 2010, 06:01
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If each year the population of the country grows by 20%, how many years will elapse before the population of the country doubles?

A) 3
B) 4
C) 5
D) 6
E) 7

OA is B and OE is:
Let the starting population be X. There will be change in population by a factor of \frac{6}{5} every year. So, in 4 years the population becomes:
(\frac{6}{5})^{4x} = 2.0736X

So, after 4 yrs, population become >double X.

I have a doubt: The population becomes double before the completion of 4th yr. So, it mean 3 yrs are passed not 4.

Experts please comment.
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Re: m12#11 - Number System [#permalink] New post 12 Jun 2010, 06:18
there is no such answer as 3.5,...instead, there are integers. it takes more than 3 years to double the population, therefore, it takes approximately 4 years to double it.
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Re: m12#11 - Number System [#permalink] New post 12 Jun 2010, 06:39
I think you misread the question stem:
If each year the population of the country grows by 20%, how many years will elapse before the population of the country doubles?

I think before completion of 4th yr, population gets doubled. So, 3 yr elapsed.
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Re: m12#11 - Number System [#permalink] New post 17 Apr 2011, 15:47
I know its been a while since someone has talked about this problem, but I agree with the author of this post. The population is doubled before the completion of the 4th year, so the answer should be 3.
Re: m12#11 - Number System   [#permalink] 17 Apr 2011, 15:47
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