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Rs. 9800 are invested partly in 9% stock at 75 and 10% stock at 80 to have equal amount of incomes. The investment in 9% stock is:

  • A.Rs. 4800
  • B.Rs. 5000
  • C.Rs. 5400
  • D.Rs. 5600

Answer: B

In stock problems, a "\( r\% \) stock at \( m \)" means Rs. 100 of face value costs Rs. \( m \) and pays Rs. \( r \) a year, so the yield on money invested is \( \tfrac{r}{m} \).

\[ \text{9\% at 75 yields } \frac{9}{75} = 12\%, \qquad \text{10\% at 80 yields } \frac{10}{80} = 12.5\% \]

Equal incomes from the two parts:

\[ 0.12x = 0.125(9800 - x) \]

\[ 0.12x = 1225 - 0.125x \quad\Longrightarrow\quad 0.245x = 1225 \]

\[ x = \text{Rs. } 5000 \]

Each part then yields Rs. 600.

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