Tk. 9800 are invested partly in 9% stock at 75 and 10% stock at 80 to have equal amounts of income. The investment in 9% stock is -

Assume that the face value of both stocks is Tk. 100 as it is not given.
Hence, the dividend per share in 1st case is Tk. 9
and dividend per share in 2nd case is Tk. 10

Let investment in 9% stock = Tk. x
and investment in 10% stock = Tk. (9800-x)

The market value of a 9% stock= Tk. 75
Number of 9% stocks he can purchase with Tk. x = x/75
Total dividend from 9% stocks = (x × 9)/75

Market value of a 10% stock = Tk. 80
Number of 10% stocks he can purchase with Tk. (9800 - x) = (9800 - x)/80
Total dividend from 10% stocks = {(9800 - x) × 10}/80

Incomes from both of the investments is same
(x × 9)/75 = {(9800 - x) × 10}/80
⇒ (x × 3)/25 = (9800 - x)/8
⇒ 24x = 25 × 9800 - 25x
⇒ 49x = 25 × 9800
⇒ x = (25 × 9800)/49
⇒ x = Tk. 5000