Discussion

In 40 years an amount becomes 6 times the original amount. What is the rate of interest?

Answer: A

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

Becoming 6 times means the interest is 5 times the principal.

\[ 5P = \frac{P \times R \times 40}{100} \]

\[ 5 = \frac{40R}{100} \quad\Longrightarrow\quad R = 12.5\% \]

The multiple counts the amount, so the interest is one less than it - using 6 instead of 5 would give 15%.

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