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# Boomtown urban planners expect the city’s population to

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Boomtown urban planners expect the city’s population to [#permalink]

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03 Mar 2012, 13:57
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45% (medium)

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63% (02:39) correct 37% (01:19) wrong based on 157 sessions

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Boomtown urban planners expect the city’s population to increase by 10% per year over the next two years. If that projection were to come true, the population two years from now would be exactly double the population of one year ago. Which of the following is closest to the percent population increase in Boomtown over the last year?

A. 20%
B. 40%
C. 50%
D. 65%
E. 75%

This how I am trying to solve, but I am stuck after this.

If we assume that today’s population is 100, next year it would be 1.1 × 100 = 110, and the following year it would be 1.1 × 110 = 121. If this is double the population of one year ago, the population at that time must have been 0.5 × 121 = 60.5. Because the problem seeks the “closest” answer choice, we can round 60.5 to 60.

Will it be 121-60/60? But this doesn't give me the right answer. Please help.
[Reveal] Spoiler: OA

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Re: Boomtown urban planners expect the city’s population to [#permalink]

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03 Mar 2012, 14:23
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enigma123 wrote:
Boomtown urban planners expect the city’s population to increase by 10% per year over the next two years. If that projection were to come true, the population two years from now would be exactly double the population of one year ago. Which of the following is closest to the percent population increase in Boomtown over the last year?

A. 20%
B. 40%
C. 50%
D. 65%
E. 75%

This how I am trying to solve, but I am stuck after this.

If we assume that today’s population is 100, next year it would be 1.1 × 100 = 110, and the following year it would be 1.1 × 110 = 121. If this is double the population of one year ago, the population at that time must have been 0.5 × 121 = 60.5. Because the problem seeks the “closest” answer choice, we can round 60.5 to 60.

Will it be 121-60/60? But this doesn't give me the right answer. Please help.

Population now - 100;
Population one year from now - 110;
Population two years from now - 121;

Since the population two years from now (121) is exactly double the population one year ago then the population one year ago was 121/2=60.5.

Now, the question asks about the population increase over the last year, so from 60.5 (last year) to 100 (now): percent increase=difference/original*100=(100-60.5)/60.5*100=39.5/60.5*100=~2/3*100=~65%.

Answer: D.

Hope it's clear.
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Re: Boomtown urban planners expect the city’s population to [#permalink]

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10 Apr 2014, 00:52
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Re: Boomtown urban planners expect the city’s population to [#permalink]

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10 Apr 2014, 02:34
enigma123 wrote:
Boomtown urban planners expect the city’s population to increase by 10% per year over the next two years. If that projection were to come true, the population two years from now would be exactly double the population of one year ago. Which of the following is closest to the percent population increase in Boomtown over the last year?

A. 20%
B. 40%
C. 50%
D. 65%
E. 75%

This how I am trying to solve, but I am stuck after this.

If we assume that today’s population is 100, next year it would be 1.1 × 100 = 110, and the following year it would be 1.1 × 110 = 121. If this is double the population of one year ago, the population at that time must have been 0.5 × 121 = 60.5. Because the problem seeks the “closest” answer choice, we can round 60.5 to 60.

Will it be 121-60/60? But this doesn't give me the right answer. Please help.

You have the calculation correct till 60.5 but we need the percentage increase of 60.5 to 100.00
=> (100 - 60.5)/60.5 = 65%. even if you round 60.5 to 60 you will reach the correct answer.
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Re: Boomtown urban planners expect the city’s population to [#permalink]

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14 Apr 2014, 01:26
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Look at the table below:

Percentage increase

$$= 40 * \frac{100}{60}$$

= 66.66

Answer = D
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Re: Boomtown urban planners expect the city’s population to [#permalink]

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19 Apr 2015, 02:13
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: Boomtown urban planners expect the city’s population to [#permalink]

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11 Apr 2016, 06:46
Am I correct to say that if we write $$P*(\frac{11}{10})^2=2S$$ where P=present population, S= start population, so 1 year ago.
That gives $$S=\frac{121P}{200}$$ but after this, how can we relate the S to P to solve the question? Does this make sense?..

Like writing $$S*\frac{x}{100}=P$$ gives 160% what's wrong here?..

Thank you!
Re: Boomtown urban planners expect the city’s population to   [#permalink] 11 Apr 2016, 06:46
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# Boomtown urban planners expect the city’s population to

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