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# M12-11

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Math Expert
Joined: 02 Sep 2009
Posts: 51263

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15 Sep 2014, 23:46
00:00

Difficulty:

55% (hard)

Question Stats:

62% (00:49) correct 38% (01:11) wrong based on 156 sessions

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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

_________________
Math Expert
Joined: 02 Sep 2009
Posts: 51263

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15 Sep 2014, 23:46
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: $$(1.2)^3 = 1.728$$.

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Joined: 22 Feb 2014
Posts: 14
Location: Dominican Republic
Concentration: Finance, Strategy
GMAT 1: 480 Q36 V21
GMAT 2: 620 Q44 V31
GPA: 3.03
WE: Analyst (Retail Banking)

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04 Oct 2014, 15:24
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...
Current Student
Joined: 06 Mar 2014
Posts: 243
Location: India
GMAT Date: 04-30-2015

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12 Oct 2014, 16:30
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: $$(1.2)^3 = 1.728$$.

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.
Senior Manager
Joined: 15 Sep 2011
Posts: 326
Location: United States
WE: Corporate Finance (Manufacturing)

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28 Jul 2015, 14:11
1
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: $$(1.2)^3 = 1.728$$.

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.

Kr,
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Joined: 28 Feb 2015
Posts: 17
Location: India
GPA: 4
WE: Consulting (Consulting)

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31 Aug 2015, 21:51
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: $$(1.2)^3 = 1.728$$.

I agree with question raised by mejia401 and earnit.
Current Student
Joined: 12 Aug 2015
Posts: 285
Concentration: General Management, Operations
GMAT 1: 640 Q40 V37
GMAT 2: 650 Q43 V36
GMAT 3: 600 Q47 V27
GPA: 3.3
WE: Management Consulting (Consulting)

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24 Sep 2015, 10:13
1

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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KUDO me plenty

Intern
Joined: 25 Jan 2013
Posts: 4

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14 Nov 2015, 23:02
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.
Intern
Joined: 14 Jun 2014
Posts: 7

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06 Mar 2016, 09:33
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
Intern
Joined: 04 Apr 2016
Posts: 2

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03 Jul 2016, 20:01
I think this is a poor-quality question and the explanation isn't clear enough, please elaborate.
Intern
Joined: 30 Jun 2016
Posts: 3

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31 Jul 2016, 12:18
yea it should be 3. After 4 years it already doubled.
Intern
Joined: 29 Oct 2015
Posts: 2
GMAT 1: 690 Q44 V40

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31 Jul 2016, 21:55
I also feel it should be 3 as the questions asks "Elapsed". brunel?
Intern
Joined: 27 Oct 2013
Posts: 27
Schools: Erasmus '17

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12 Aug 2016, 18:21
1
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.
Intern
Joined: 19 Jun 2016
Posts: 15

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19 Aug 2016, 03:18
2
I would like to second the opinion expressed by others.

Question asks for number of elapsed years and not by the years.
Manager
Joined: 19 Jul 2015
Posts: 61
Location: India
GMAT 1: 720 Q51 V35
GPA: 3.69

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04 Oct 2016, 10:14
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.
Intern
Joined: 13 Jul 2016
Posts: 37
GMAT 1: 770 Q50 V44

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01 Nov 2016, 22:02
I too think that the answer should be 3, not 4
Intern
Joined: 17 Aug 2016
Posts: 18

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01 Apr 2017, 03:22
Bunuel

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

shailabh
Intern
Joined: 02 Feb 2016
Posts: 27
Location: United States
GMAT 1: 710 Q49 V38
GPA: 3.5

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26 Apr 2017, 19:13
I marked 4, but I think the question is ambiguous.
Intern
Joined: 19 Jul 2010
Posts: 6

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26 Jun 2017, 09:29
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.
Intern
Joined: 17 Jan 2017
Posts: 1
Location: India

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12 Jul 2017, 23:49
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!
Re: M12-11 &nbs [#permalink] 12 Jul 2017, 23:49

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