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# A sum of money doubles itself in 7 years. In how many years it becomes

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Senior Manager
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Joined: 10 Oct 2018
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A sum of money doubles itself in 7 years. In how many years it becomes  [#permalink]

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11 Apr 2019, 06:44
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15% (low)

Question Stats:

75% (00:30) correct 25% (01:05) wrong based on 36 sessions

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A sum of money doubles itself in 7 years. In how many years it becomes four fold?
(A) 14 years
(B) 21 years
(C) 28 years
(D) 35 years
(E) 49 years

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Re: A sum of money doubles itself in 7 years. In how many years it becomes  [#permalink]

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11 Apr 2019, 07:09
$$n$$ at $$t= 0$$.

$$2n$$ at $$t = 7$$.

$$2*2n = 4n$$ at $$t =14$$.
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Re: A sum of money doubles itself in 7 years. In how many years it becomes  [#permalink]

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14 Apr 2019, 18:45
nm97 wrote:
A sum of money doubles itself in 7 years. In how many years it becomes four fold?
(A) 14 years
(B) 21 years
(C) 28 years
(D) 35 years
(E) 49 years

If the initial amount of money is x dollars, then 7 years later, it will be 2x dollars, and in another 7 years, it will be 4x dollars. Thus, it takes 14 years to quadruple the initial amount money.

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Re: A sum of money doubles itself in 7 years. In how many years it becomes  [#permalink]

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29 Apr 2019, 06:23
nm97 wrote:
A sum of money doubles itself in 7 years. In how many years it becomes four fold?
(A) 14 years
(B) 21 years
(C) 28 years
(D) 35 years
(E) 49 years

Can anyone explain this question. Simple Interest will be applied to it right? With that the answer is 21 years.
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Re: A sum of money doubles itself in 7 years. In how many years it becomes  [#permalink]

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29 Apr 2019, 10:43
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MG1105 It isn't right. You are supposing the amount of money linearly increases, but it's not the case (it's quadratic).

This is what you say:

x at t = 0
2x at t = 7
3x at t = 14
4x at t = 21

But the sum of money doubles itself in 7 years! Hence, if we have 2x after 7 years, we'll get its double after the same amount of time (4x at t = 14). Hope it's clear.
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Re: A sum of money doubles itself in 7 years. In how many years it becomes   [#permalink] 29 Apr 2019, 10:43
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