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A certain sum doubles in 7 years at simple interest. The same sum under the same interest rate will become 4 times in how many years ?

A. 14 years B. 28 years C. 21 years D. 10 years Answer: Option C
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Solution(By Apex Team)

$\begin{array}{l} \frac{(n-1)}{t_{1}}=\frac{(m-1)}{t_{2}} \\ \frac{1}{7}=\frac{3}{t_{2}} \\ t_{2}=21 \text { years } \end{array}$