Rs. 9800 is invested partly in 9% stock at 75 and partly in 10% stock at 80 so that the incomes are equal. How much is invested in the 9% stock?

Let

• x = amount invested in 9% at 75; then (9800 − x) in 10% at 80.

Income from each part

• Nominal bought from x: (x/75) × 100; annual income = 9% of this = 9x/75 = 3x/25.
• Nominal bought from (9800 − x): ((9800 − x)/80) × 100; income = 10% of this = (9800 − x)/8.

Set incomes equal and solve
3x/25 = (9800 − x)/824x = 25(9800 − x) ⇒ 24x = 245000 − 25x49x = 245000 ⇒ x = 5000

Verification
Income from 9% part = 3×5000/25 = 600; from 10% part = (9800−5000)/8 = 4800/8 = 600 (equal).

Final Answer
Rs. 5,000