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# A sum of money doubles itself at a compound interest in 15 y

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Math Expert
Joined: 02 Sep 2009
Posts: 39744
Re: A sum of money doubles itself at a compound interest in 15 y [#permalink]

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07 Feb 2014, 03:56
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aimlockfire1 wrote:
A sum of money doubles itself at a compound interest in 15 years . In how may years it will become 8 times ??

a) 30 years

b) 40 years

c) 45 years

d) 50 years

e) 60 years

If the sum doubles (x2) in 15 years, then it to become 2*2*2 = 8 times as large it should double three times, so three 15 years time period are needed, which is 3*15 = 45 years.

[Reveal] Spoiler: OA

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Re: A sum of money doubles itself at a compound interest in 15 y [#permalink]

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11 Aug 2014, 01:44
1
KUDOS
Let's say amount of money = $2, then 8 times = 16$

Then
Year 00 = 2 $Year 15 = 4$
Year 30 = 8 $Year 45 = 16$ --> this is our answer, hence C.
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A sum of money doubles itself at a compound interest in 15 y [#permalink]

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13 Aug 2014, 02:15
Say initial amount = 100

Final amount = 800

100 >>>>>>>> 200 >>>>>>>>> 400 >>>>>>>>>> 800

........... 15 ............ 15 ................. 15

Total = 15*3 = 45

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Re: A sum of money doubles itself at a compound interest in 15 y [#permalink]

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05 Jul 2016, 12:25
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: A sum of money doubles itself at a compound interest in 15 y [#permalink]

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21 Jun 2017, 05:18
Bunuel wrote:
aimlockfire1 wrote:
A sum of money doubles itself at a compound interest in 15 years . In how may years it will become 8 times ??

a) 30 years

b) 40 years

c) 45 years

d) 50 years

e) 60 years

If the sum doubles (x2) in 15 years, then it to become 2*2*2 = 8 times as large it should double three times, so three 15 years time period are needed, which is 3*15 = 45 years.

If the lump sum is 8x then this represents a 2x^3 increase to

x(1 +r)^15(3)= 8x

Thus
"C"
Re: A sum of money doubles itself at a compound interest in 15 y   [#permalink] 21 Jun 2017, 05:18
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