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The true discount on Rs. 1760 due after a certain time at 12% per annum is Rs. 160. The time after which it is due is:

  • A.6 months
  • B.8 months
  • C.9 months
  • D.10 months

Answer: D

The present worth (PW) is what the sum is really worth today; the true discount (TD) is the interest that PW would earn over the remaining time:

\[ \text{Amount} = \text{PW}\left(1 + \frac{RT}{100}\right), \qquad \text{TD} = \text{Amount} - \text{PW} \]

\[ \text{PW} = 1760 - 160 = 1600 \]

The true discount is simple interest on the present worth, so

\[ 160 = \frac{1600 \times 12 \times T}{100} = 192\,T \]

\[ T = \frac{160}{192} = \frac{5}{6} \text{ year} = 10 \text{ months} \]

Note the interest is charged on the present worth of Rs. 1600, not on the Rs. 1760 face value - using the face value gives about 9 months, which is why that option is listed.

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