A principal becomes twice of its amount in 20 yrs at a certain rate of simple interest. At the same rate of simple interest, that principal becomes thrice of its amount in (a) 30 years (b) 35 years (c) 40 years (d) 45 years

(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