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# m012#11

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Manager
Joined: 11 Apr 2011
Posts: 109
Followers: 0

Kudos [?]: 26 [0], given: 16

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26 Jul 2011, 04:04
If each year the population of the country grows by 20%, how many years will elapse before the population of the country doubles?

3
4
5
6
7

i calculated 5 yrs using the simple interest formula.
i = prt/100

here, i = p, r=20, so, t = 5yrs.

why the explanation given and this formula yields different results? why can't the simple interest formula can be used here?
Senior Manager
Joined: 24 Mar 2011
Posts: 457
Location: Texas
Followers: 5

Kudos [?]: 137 [1] , given: 20

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26 Jul 2011, 06:01
1
KUDOS
SI formula is not correct here, every year the population grown by 20% not on original but what is compounded last year... so either you have to use compound formula -

CI = $$P (1+\frac{r}{n})^(nt)$$

CI = 2P, n =1, r = 0.2,

2P =$$P (1+\frac{0.2}{1})^(1*t)$$

==> t=4

another way -

since this is simple calculation, i would just do the calculation -
initially - 100 people
1st year - 120 people
2nd year - 144 people
3rd year - 1.2*144 people
4th year = 1.44*144 ~ 210 people...
Intern
Status: Don't worry about finding inspiration. It eventually come >>>>>>>
Joined: 31 May 2011
Posts: 23
Location: Î Ñ D Ï Â
Followers: 0

Kudos [?]: 9 [0], given: 2

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26 Jul 2011, 11:24
Simple interest formula is not used here because no. of people of that country is continuously vary every year.
Let population be 100. now on
1st year = 120
2nd year = 144
3rd year = 172.8
4th year = 207.36
thus it will cross the double population landmark on 4th year.
Re: m012#11   [#permalink] 26 Jul 2011, 11:24
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# m012#11

Moderator: Bunuel

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