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The difference between simple and compound interests compounded annually on a certain sum of money for 2 years at 4% per annum is Re. 1. The sum (in Rs.) is:

  • A.625
  • B.630
  • C.640
  • D.650

Answer: A

\[ A = P\left(1 + \frac{R}{100}\right)^{n}, \qquad \text{CI} = A - P, \qquad \text{SI} = \frac{PRn}{100} \]

\[ \text{CI} - \text{SI} = P\left(\frac{R}{100}\right)^{2} \]

\[ 1 = P(0.04)^{2} = 0.0016P \quad\Longrightarrow\quad P = \text{Rs. } 625 \]

Checking: simple interest is Rs. 50 and compound interest Rs. 51. The Re. 1 gap is 4% of the Rs. 25 earned in the first year, which is exactly what the squared-rate formula encodes.

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