Discussion

In how many years will a sum of money doubles itself at 5% per annum on simple interest?

Answer: B

\[ \text{SI} = \frac{P \times R \times T}{100} \]

Doubling means the interest equals the principal.

\[ P = \frac{P \times 5 \times T}{100} \quad\Longrightarrow\quad 100 = 5T \]

\[ T = 20 \text{ years} \]

The general rule for doubling under simple interest is \( T = \tfrac{100}{R} \).

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