Discussion

At what rate percent on simple interest will a sum of money double itself in 30 years?

Answer: A

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

Doubling means the interest equals the principal.

\[ P = \frac{P \times R \times 30}{100} \quad\Longrightarrow\quad 100 = 30R \]

\[ R = \frac{10}{3} = 3\tfrac{1}{3}\% \]

The principal cancels, so the answer holds for any sum.

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