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The annual birth and death rate in a country per 1000 are 39.4 and 19.

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The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 20 Oct 2010, 03:43
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The annual birth and death rate in a country per 1000 are 39.4 and 19.4 respectively . the number of years in which the population would be doubled assuming there is no emigration or immigration is

A. 20
B. 25
C. 30
D. 35
E. 40
[Reveal] Spoiler: OA
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 20 Oct 2010, 04:03
anilnandyala wrote:
the annual birth and death rate in a country per 1000 are 39.4 and 19.4 respectively . the number of years in which the population would be doubled assuming there is no emigration
or immigration is
a 20
b 25
c 30
d 35
e 40


Suppose the population of the country in current year is 1000.
So annual increase is 1000 + 39.4 - 19.4=1020
Hence every year there is an increase of 2%.

2000=1000(1+(2/100))^n

n=35 Answer is D. (But Calculation is somewhat tedious.) Anything simple than this?

Consider KUDOS if its helpful.
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 20 Oct 2010, 04:28
ankitranjan wrote:
Suppose the population of the country in current year is 1000.
So annual increase is 1000 + 39.4 - 19.4=1020
Hence every year there is an increase of 2%.

2000=1000(1+(2/100))^n

n=35 Answer is D. (But Calculation is somewhat tedious.) Anything simple than this?

Consider KUDOS if its helpful.


Yes, I also solved it in that way. The problem is that you cannot use a scientific calculator during the exam LOL.
I think we need the help of the moderators!
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 20 Oct 2010, 07:41
Could you please advise if there are any easier ways to solve this?
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 20 Oct 2010, 08:08
didn't understand the question
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 20 Oct 2010, 13:10
I stopped when I realized how long it would take me to calculate. But 35 seems good enough for me
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 21 Oct 2010, 17:03
LENGTHY CALCN :(
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 17 Mar 2015, 10:46
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An easier method is the Rule of 70 which is a simple way to calculate the approximate number of years it takes for the level of a variable growing at a constant rate to double. This rule states that the approximate number of years n for a variable growing at the constant growth rate of R percent, to double is

n = 70/R

Since B-D = 39.4-19.4 = 20 normalized to 1000 = 2%

The n = 70/2 = 35
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 17 Mar 2015, 12:25
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kcr2210 wrote:
An easier method is the Rule of 70 which is a simple way to calculate the approximate number of years it takes for the level of a variable growing at a constant rate to double. This rule states that the approximate number of years n for a variable growing at the constant growth rate of R percent, to double is

n = 70/R

Since B-D = 39.4-19.4 = 20 normalized to 1000 = 2%

The n = 70/2 = 35


wow! I have never heard about this, interesting...
Do you have any support to prove this approach will always show me the right answer?

Thank you for your reply.
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 17 Mar 2015, 19:49
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guijob wrote:
kcr2210 wrote:
An easier method is the Rule of 70 which is a simple way to calculate the approximate number of years it takes for the level of a variable growing at a constant rate to double. This rule states that the approximate number of years n for a variable growing at the constant growth rate of R percent, to double is

n = 70/R

Since B-D = 39.4-19.4 = 20 normalized to 1000 = 2%

The n = 70/2 = 35


wow! I have never heard about this, interesting...
Do you have any support to prove this approach will always show me the right answer?

Thank you for your reply.


You can use the n = 70/r rule whenever you want the amount to double in an annual compounded interest rate scenario but GMAT doesn't expect you to know this formula so the calculations will either be simpler or the formula will be given in the question.
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 18 Mar 2015, 21:29
kcr2210 wrote:
An easier method is the Rule of 70 which is a simple way to calculate the approximate number of years it takes for the level of a variable growing at a constant rate to double. This rule states that the approximate number of years n for a variable growing at the constant growth rate of R percent, to double is

n = 70/R

Since B-D = 39.4-19.4 = 20 normalized to 1000 = 2%

The n = 70/2 = 35



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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 16 Apr 2015, 02:30
metallicafan wrote:
ankitranjan wrote:
Suppose the population of the country in current year is 1000.
So annual increase is 1000 + 39.4 - 19.4=1020
Hence every year there is an increase of 2%.

2000=1000(1+(2/100))^n

n=35 Answer is D. (But Calculation is somewhat tedious.) Anything simple than this?

Consider KUDOS if its helpful.


Yes, I also solved it in that way. The problem is that you cannot use a scientific calculator during the exam LOL.
I think we need the help of the moderators!




Can anyone guide me how to solve this manually.I don't have any clue.

2000=1000(1+(2/100))^n
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 16 Apr 2015, 23:05
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ssriva2 wrote:
metallicafan wrote:
ankitranjan wrote:
Suppose the population of the country in current year is 1000.
So annual increase is 1000 + 39.4 - 19.4=1020
Hence every year there is an increase of 2%.

2000=1000(1+(2/100))^n

n=35 Answer is D. (But Calculation is somewhat tedious.) Anything simple than this?

Consider KUDOS if its helpful.


Yes, I also solved it in that way. The problem is that you cannot use a scientific calculator during the exam LOL.
I think we need the help of the moderators!


Can anyone guide me how to solve this manually.I don't have any clue.

2000=1000(1+(2/100))^n


You cannot solve it manually. You need a calculator to do it.
Though, when the amount is twice the principal, we have a simple formula (which will be given in the question if this question comes in GMAT)
The principal doubles in 70/r years.
Since the rate of interest is 2% here, and the principal doubles from 1000 to 2000, the number of years it will take is 70/2 = 35 years.
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Re: The annual birth and death rate in a country per 1000 are 39.4 and 19. [#permalink] New post 17 Apr 2015, 03:35
VeritasPrepKarishma wrote:
ssriva2 wrote:
metallicafan wrote:
Suppose the population of the country in current year is 1000.
So annual increase is 1000 + 39.4 - 19.4=1020
Hence every year there is an increase of 2%.

2000=1000(1+(2/100))^n

n=35 Answer is D. (But Calculation is somewhat tedious.) Anything simple than this?

Consider KUDOS if its helpful.


Yes, I also solved it in that way. The problem is that you cannot use a scientific calculator during the exam LOL.
I think we need the help of the moderators!


Can anyone guide me how to solve this manually.I don't have any clue.

2000=1000(1+(2/100))^n


You cannot solve it manually. You need a calculator to do it.
Though, when the amount is twice the principal, we have a simple formula (which will be given in the question if this question comes in GMAT)
The principal doubles in 70/r years.
Since the rate of interest is 2% here, and the principal doubles from 1000 to 2000, the number of years it will take is 70/2 = 35 years.[/quote]

Thanks a lot Karishma.I usually get struck on this last part in these kind of questions.
Re: The annual birth and death rate in a country per 1000 are 39.4 and 19.   [#permalink] 17 Apr 2015, 03:35
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