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nm97
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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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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