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M12-11

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M12-11 [#permalink]

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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
[Reveal] Spoiler: OA

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New post 16 Sep 2014, 00:46
Official Solution:

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

In 4 years the population, \(X\), of the country will grow to \((1.2)^{4}X = (1.44)^{2}X = 2.0736X\). 3 years is not enough: \((1.2)^3 = 1.728\).

Answer: B
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Re: M12-11 [#permalink]

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New post 04 Oct 2014, 16:24
The question asks for how many years will elapse before the population of the country doubles? In 4 years the population will already be doubled. Shouldn't it be 3 years the correct answer? Please, help me undertand this...

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Re: M12-11 [#permalink]

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New post 12 Oct 2014, 17:30
Bunuel wrote:
Official Solution:

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

In 4 years the population, \(X\), of the country will grow to \((1.2)^{4}X = (1.44)^{2}X = 2.0736X\). 3 years is not enough: \((1.2)^3 = 1.728\).

Answer: B


Since after 3 years population is not exactly double but close to it so clearly during the 4th year it would double and at t=4 exact value would be more than double so why isn't 3 the right answer as to be true to the question..3 years have passed and it is during the 4th year that population doubles.

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Re: M12-11 [#permalink]

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New post 28 Jul 2015, 15:11
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earnit wrote:
Bunuel wrote:
Official Solution:

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

In 4 years the population, \(X\), of the country will grow to \((1.2)^{4}X = (1.44)^{2}X = 2.0736X\). 3 years is not enough: \((1.2)^3 = 1.728\).

Answer: B


Since after 3 years population is not exactly double but close to it so clearly during the 4th year it would double and at t=4 exact value would be more than double so why isn't 3 the right answer as to be true to the question..3 years have passed and it is during the 4th year that population doubles.



Even though the population doubled during the 4th year, only three years have elapsed. Elapsed literally means what "has passed", and with how the questions is worded, since the fourth year has not passed, IMO A.

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Re: M12-11 [#permalink]

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New post 31 Aug 2015, 22:51
Bunuel wrote:
Official Solution:

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

In 4 years the population, \(X\), of the country will grow to \((1.2)^{4}X = (1.44)^{2}X = 2.0736X\). 3 years is not enough: \((1.2)^3 = 1.728\).

Answer: B


I agree with question raised by mejia401 and earnit.

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Re: M12-11 [#permalink]

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New post 24 Sep 2015, 11:13
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Answer: B.

If we take the number of population now as 100 then

(1) - 120
(2) - 144
(3) - 172.8
(4) - obviosly 20% of 172.8 is greater than 30 which gives us 2X100 hence in year 4 the population doubles
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Re M12-11 [#permalink]

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New post 15 Nov 2015, 00:02
I think this is a poor-quality question and I don't agree with the explanation. Hi,
Acoording to wording of the problem- "how many years will elapse before the population of the country doubles?"- the answer should be 3 and not 4. So, either change the Answer or change the question wording.

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Re M12-11 [#permalink]

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New post 06 Mar 2016, 10:33
I think this is a high-quality question and I agree with explanation. there is another nice and fast solution that can be applied here : there is rule of thumb that says that in order to find out the number of years needed for an amount to double you divide 70-72 to the growth rate => 70/20=3.5 so 4 years as 3 are not enough

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Re M12-11 [#permalink]

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New post 03 Jul 2016, 21:01
I think this is a poor-quality question and the explanation isn't clear enough, please elaborate.

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New post 31 Jul 2016, 13:18
yea it should be 3. After 4 years it already doubled.

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New post 31 Jul 2016, 22:55
I also feel it should be 3 as the questions asks "Elapsed". brunel?

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New post 12 Aug 2016, 19:21
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For all who are choosing or discussing or arguing for A(3).........

Till Year 2000, population is 100 .

Year 2001: Population becomes 120 ............. 1 Year elapsed
Year 2002: Population becomes 144 ............. 2 Year elapsed

Year 2003: Population becomes 172 ............. 3 Year elapsed

Year 2004: Population > 200 .................. 4 year elapsed

Nice discussion, although i got right ans, in first attempt, but never gave thought to this much intricate details.

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New post 19 Aug 2016, 04:18
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I would like to second the opinion expressed by others.

Question asks for number of elapsed years and not by the years.

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Re M12-11 [#permalink]

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New post 04 Oct 2016, 11:14
I think this is a high-quality question and I don't agree with the explanation. I think answer should be 3 as stated by others above.
Bunuel please help.

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Re: M12-11 [#permalink]

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New post 01 Nov 2016, 23:02
I too think that the answer should be 3, not 4

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Re: M12-11 [#permalink]

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New post 01 Apr 2017, 04:22
Bunuel

Request your response pls. I agree with the rest, in that I marked 3 years as the answer.

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Re: M12-11 [#permalink]

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New post 26 Apr 2017, 20:13
I marked 4, but I think the question is ambiguous.

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Re: M12-11 [#permalink]

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New post 26 Jun 2017, 10:29
Leaving the argument for a about elapsed time aside. A usefull way to solve this question can be by the RULE OF 72- which say at what rate will the any amount or x double .

So divide 72/20 (ie the rate here) = 3.6 years.

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Re: M12-11 [#permalink]

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New post 13 Jul 2017, 00:49
Took population as 100! for it to double it means it should become 200.
1.now percentage increase from 100 to 200 is 100%
2.now divide 100% by 20% to get 5 years
3.the question asks how many years will elapse before the population of the country doubles?
so 5-1= 4 years.
I dont know if my approach is correct!

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Re: M12-11   [#permalink] 13 Jul 2017, 00:49

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