121.
The difference between the simple interest received from two different sources on Rs. 1500 for 3 years is Rs. 13.50. The difference between their rates of interest is?
Answer: C
\[ \text{SI} = \frac{P \times R \times T}{100} \]
The sum and the period are common, so only the gap in rates produces the difference.
\[ \frac{1500 \times \Delta R \times 3}{100} = 13.50 \]
\[ 45\,\Delta R = 13.50 \quad\Longrightarrow\quad \Delta R = 0.3\% \]
Neither rate needs to be known individually - only their difference enters the calculation.
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