A sum of money placed at compound interest doubles itself in 5 years in how many years it would to 8 times of itself at the same rate of interest?

A. 7 years
B. 10 years
C. 15 years
D. 20 years

Explanation:

Let principal be Rs x and the rate is R % p.a.

Then X × (1 + R/100)⁵ = 2x
=> (1 + R/100)⁵ = 2
Let x × (1 + R/100)^t = 8x
=> (1 + R/100)^t = 8 = 2³
={(1 + R/100)⁵ }³
=> (1 + R/100)^t = (1 + R/100)¹⁵
=> T = 15 Years

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