Show A sum of money placed at Compound Interest (C.I.) doubles itself in 5 years. It will amount to eight times itself at the same rate of interest in:-A. 7 yearsB. 10 yearsC. 15 yearsD. 20 yearsAnswer VerifiedHint: To find the number of years in which Compound Interest amounts to eight times given that a sum of money placed at Compound Interest (C.I.) doubles itself in 5 years. We shall take the sum of the money, i.e. the principal amount be ‘X’. Now, according to the question, after 5 years, X becomes 2X, and after next five years, i.e. after 10 years, 2X will become 4X because of the same, and so on. We will use \[A\ \ =\ \ P\ {{\left( 1\ \ +\ \ \dfrac{R}{100} \right)}^{n}}\] . Let us take \[A\] as $2X$ , $P$ as $X$ and $n$ as $5$ . We shall put these values in the equation to find the answer. To find the number of years in which the sum that is given to us will become 8 times of itself, take \[A\] as $8X$ , $P$ as $X$ . After substitution and comparing both the equations, we will get the number of years. Complete step by step answer: So, the correct answer is “Option C”. Note: Be careful with the equation of CI \[A\ \ =\ \ P\ {{\left( 1\ \ +\ \ \dfrac{R}{100}
\right)}^{n}}\] . There can be a chance of making an error in this equation. We can also use the following method to find the answer to this question.
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