Question

A certain sum lent out at simple interest doubles itself in $20$ years. Number of years in which this sum trebles itself at the same rate of interest is:

• A. $30$
• B. $40$
• C. $25$
• D. $20$

Answer 1

The correct option is B $40$

Let the amount doubles in $20$ years @ $R\%$ interest

Then $100=$ $\frac{100\times 20\times R}{100}\Rightarrow R=5$ $\%$

Let the amount trebles in $T$ years @ $5\%$.

Then $200=$ $\frac{100\times 5\times T}{100}\Rightarrow T=40$ years