Discussion
Rohit invests a sum of money at compound interest which doubles itself in 3 years, find in how many years will it amount to four times itself ?
Answer: B
Compound growth multiplies, so two doubling periods give a quadrupling.
\[ \left(1 + \frac{R}{100}\right)^{3} = 2 \quad\Longrightarrow\quad \left(1 + \frac{R}{100}\right)^{6} = 2^{2} = 4 \]
So it takes 6 years. The rate itself is never needed.
Under simple interest the answer would instead be 9 years, since there the interest earned each year stays constant.
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