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population manhattan word problem

Expert replies
by viny » Sat Jul 27, 2013 10:23 pm
The population of grasshoppers doubles in a particular field every year. Approximately how many years will it take the population to grow from 2,000 grasshoppers to 1,000,000 or more?

i am trying to solve it using geometric progression where b=a *(r^(n-1)) :where b is the nth term and a is the first
i am getting 1000000=2000(2^n-1)
which on solving gives me 2^(n-1)=500
i want to know how should i convert 500 to the power of 2 in order to easily solve for n.what i did basically is i know 2^9 is 512 which is approximately equal to 500 so i considered my new eqn to be
2^n-1 = 2^9 which gives me n value to be 10.
but the real answer is 9

please help me on where i am going wrong and also a better way to convert a number to the nearest exponent like here i converted 500 in power of 2 as 2^9 ,is there any method to do the same.
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Source: — Problem Solving |

by Brent@GMATPrepNow » Sat Jul 27, 2013 10:31 pm
viny wrote:The population of grasshoppers doubles in a particular field every year. Approximately how many years will it take the population to grow from 2,000 grasshoppers to 1,000,000 or more?

i am trying to solve it using geometric progression where b=a *(r^(n-1)) :where b is the nth term and a is the first
i am getting 1000000=2000(2^n-1)
which on solving gives me 2^(n-1)=500
i want to know how should i convert 500 to the power of 2 in order to easily solve for n.what i did basically is i know 2^9 is 512 which is approximately equal to 500 so i considered my new eqn to be
2^n-1 = 2^9 which gives me n value to be 10.
but the real answer is 9

please help me on where i am going wrong and also a better way to convert a number to the nearest exponent like here i converted 500 in power of 2 as 2^9 ,is there any method to do the same.
It should be 2000(2^n) not 2000(2^n-1).
To see why, let's draw a table and look for a pattern.

Year 0: population = 2000
Year 1: population = 2000(2)
Year 2: population = 2000(2)(2) = 2000(2^2)
Year 3: population = 2000(2)(2)(2) = 2000(2^3)
Year 4: population = 2000(2)(2)(2)(2) = 2000(2^4)
.
.
.
Year n: population = 2000(2^n)

So, we get the following equation: 2000(2^n) = 1,000,000
Divide both sides by 2000 to get 2^n = 500
2^8 = 256 (nope)
2^9 = 512 (bingo)

n = 9

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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viny wrote: ↑
Sat Jul 27, 2013 10:23 pm
The population of grasshoppers doubles in a particular field every year. Approximately how many years will it take the population to grow from 2,000 grasshoppers to 1,000,000 or more?

Solution:

We can just tabulate the population of grasshoppers as follows:

Year 0: 2,000

Year 1: 4,000

Year 2: 8,000

Year 3: 16,000

Year 4: 32,000

Year 5: 64,000

Year 6: 128,000

Year 7: 256,000

Year 8: 512,000

Year 9: 1,024,000

We see that it takes 9 years for the population of 2,000 grasshoppers to grow to 1,000,000 or more grasshoppers.

Answer: 9

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