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Age of the Earth

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Age of the Earth

by alex.gellatly » Wed Aug 08, 2012 8:29 pm
The age of the Earth is approximately 1.3 *10^17 seconds, and one year is approximately 3.2*10^7 seconds. Which of the following is closest to the age of the Earth in years?

2.5*10^9
4.1*10^9
1.9*10^10
2.5*10^11
4.1*10^11

I know these questions should be easy.... but I always seem to have trouble. What is a good approach for setting them up? Thanks
A useful website I found that has every quant OG video explanation:

https://www.beatthegmat.com/useful-websi ... tml#475231
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by truplayer256 » Wed Aug 08, 2012 8:35 pm
1 year = 3.2*10^7 seconds
Age of earth = 1.3*10^17 seconds
Age of earth in years = (1.3*10^17 seconds) * (1 year)/(3.2*10^7 seconds) = (1.3/3.2) * 10^10
= About 3.9 * 10^9 which most closely resembles choice B.
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by GMATGuruNY » Thu Aug 09, 2012 3:27 am
alex.gellatly wrote:The age of the Earth is approximately 1.3 *10^17 seconds, and one year is approximately 3.2*10^7 seconds. Which of the following is closest to the age of the Earth in years?

2.5*10^9
4.1*10^9
1.9*10^10
2.5*10^11
4.1*10^11

I know these questions should be easy.... but I always seem to have trouble. What is a good approach for setting them up? Thanks
Set up a PROPORTION and round values to the nearest MULTIPLE OF 10.

(3.2)(10^7) seconds / 1 year = (1.3)(10^17) seconds / x years

x =

= (1.3)(10^17) / (3.2)(10^7)

= (13/32)(10^10)

= (39/96)(10^10)

≈ (40/100)(10^10)

≈ (4/10)(10^10)

≈ 4 * 10^9.

The correct answer is B.
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