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The simple interest on Rs. 750 for 2 years is equal to the true discount on Rs. 810 for the same time and at the same rate. The rate of interest per annum is:

  • A. 4%
  • B.13/3%
  • C.5%
  • D.31/6%

Answer: A

Write \( k = \dfrac{R \times 2}{100} \). Simple interest is on the full sum; true discount is on the present worth:

\[ \text{SI} = 750\,k, \qquad \text{TD} = \frac{810\,k}{1+k} \]

Setting them equal and cancelling \( k \):

\[ 750(1+k) = 810 \quad\Longrightarrow\quad 1+k = 1.08 \quad\Longrightarrow\quad k = 0.08 \]

\[ \frac{2R}{100} = 0.08 \quad\Longrightarrow\quad R = 4\% \]

The two years enter only at the last step; everything before it is independent of the period.

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