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The banker's gain on a bill due 3 year hence at 6% per annum is Rs. 18. The true discount is:

  • A.Rs.100
  • B.Rs.75
  • C.Rs.50
  • D.Rs.33.33

Answer: A

Four quantities are related throughout this topic. Writing \( k = \dfrac{RT}{100} \):

\[ \text{BD} = A\,k, \qquad \text{TD} = \frac{A\,k}{1+k} = \text{PW}\,k, \qquad \text{BG} = \text{BD} - \text{TD} = \text{TD}\,k \]

The banker's discount is interest on the face value; the true discount is interest on the present worth; the banker's gain is the interest on the true discount.

Here \( k = \dfrac{6 \times 3}{100} = 0.18 \), and the banker's gain is interest on the true discount:

\[ \text{BG} = \text{TD} \times k \quad\Longrightarrow\quad 18 = \text{TD} \times 0.18 \]

\[ \text{TD} = \frac{18}{0.18} = \text{Rs. } 100 \]

The banker's gain is always the smallest of the four quantities, being interest on the discount rather than on either principal.

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