Question
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 = ?
Answer: Option C
$$\eqalign{
& \frac{{\left( {n - 1} \right)}}{{{t_1}}} = \frac{{\left( {m - 1} \right)}}{{{t_2}}} \cr
& \frac{1}{7} = \frac{3}{{{t_2}}} \cr
& {t_2} = 21{\text{ years}} \cr} $$
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$$\eqalign{
& \frac{{\left( {n - 1} \right)}}{{{t_1}}} = \frac{{\left( {m - 1} \right)}}{{{t_2}}} \cr
& \frac{1}{7} = \frac{3}{{{t_2}}} \cr
& {t_2} = 21{\text{ years}} \cr} $$
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