(c) 40 years
Explanation
Simple Interest (I) = \frac{\text{Prt}}{100}
x = \frac{\text{x × r × 20}}{100}
or, r = \frac{\text{x × 100}}{x × 20}
or, r = 5 %
Now, 2x = \frac{\text{x × 5 × t}}{100}
or, t = \frac{\text{2x × 100}}{x × 5}
or, t = 40 years