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M01-09

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Math Expert
Joined: 02 Sep 2009
Posts: 44655

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23 Nov 2017, 21:03
Spongebob02 wrote:
Bunuel wrote:
Official Solution:

The population of Linterhast was 3,600 people in 1990 and 4,800 people in 1993. If the population growth rate per thousand is constant, then what will be the population in 1996?

A. 6,000
B. 6,400
C. 7,200
D. 8,000
E. 9,600

This is a set rate problem. If the population grew by 1,200 people in the past three years, then it grew by 33 percent:
$$\frac{4800-3600}{3600} = \frac{1200}{3600} = \frac{1}{3} \approx 33%$$

Therefore in the next three years the population will grow at the same rate of 33% because the growth rate has been constant.

$$4,800 + \frac{1}{3}*4,800 = 4,800 + 1,600 = 6,400$$

Another approach is to backsolve by comparing the ratio of each answer to 4,800. For example, the ratio of 7,200 to 4,800 is not the same as the ratio of 4,800 to 3,600.

$$4,800 + \frac{1}{3}*4,800 = 4,800 + 1,600 = 6,400$$ I did not understand this equation ?

Sent from my Redmi 3S using GMAT Club Forum mobile app

In three years from 1990 to 1993 the population grew by third. Since the growth rate is constant, then in the next three years, from 1993 to 1996, the population will also grow by third.

Hope it's clear.
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03 Dec 2017, 16:28
Spongebob02

Quote:
$$4,800 + \frac{1}{3}*4,800 = 4,800 + 1,600 = 6,400$$ I did not understand this equation ?

We have simply converted decimal (0.33%) in to fraction (1/3) for ease of calculations while dealing with fractions.
Hope this helps!
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Intern
Joined: 16 Jul 2013
Posts: 33
Location: Hungary
Concentration: Entrepreneurship, Marketing
GMAT 1: 720 Q48 V40
GPA: 3.37

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20 Dec 2017, 00:06
For me this was easier and quicker to solve using the ratios given in the question stem:

4800/3600 = x/4800
x = 6400
Intern
Joined: 25 Nov 2017
Posts: 4

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20 Dec 2017, 00:16
Easy question if ever comes up on GMAT:

We can see every 3 population grew up by 1 unit.

Divide by 400
We get the ratio 9:12

Next equivalent ratio should be (12*4)/3=16

So, 16*400=6400

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Intern
Joined: 02 Jun 2013
Posts: 6

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26 Mar 2018, 10:25
I think this is a high-quality question and I agree with explanation.
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Re M01-09   [#permalink] 26 Mar 2018, 10:25

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M01-09

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