An amount becomes double in 7 years on simple interest. The amount would be four times in ______ years on the same rate of simple interest.

Correct option is C

Given:
Time = 7 years
Formula used:
Simple interest = $$\frac{(P×R×T)}{100}$$​​
Where Principle = P , Rate of interest= R, Time = T
Solution:
Let Principle P = x
So in 7 years x became 2x.
S.I. = Amount – Principle
S.I = 2x – x
S.I = x
Now,
​$$S.I = \frac{(P×R×T)}{100} \\\ \\x =\frac{ (x×R×7)}{100} \\\ \\R = \frac{100}{7} \$$​​
For amount be 4 times , S.I will be 3x
​$$3x = \frac{(x×\frac{100}{7}×T)}{100} \\\ \\3 = \frac{T}{7}$$​​
T = 21 years