Rs. 2780 are invested party in 4% stock at 75 and 5% stock at 80 to have equal amount of incomes. The investment in 5% stock is?

Let the investment in 4% stock be Rs. N.
Then, investment in 5% stock = Rs. (2780 - N)
Income from 4% stock = Rs. (4/75) x N
Income from 5% stock = Rs. [(5/80 x (2780 - N)]
∴ 4N / 75 = (2780 - N) / 16
or N = 1500
So, investment in 5% stock = Rs. (2780 - 1500) = Rs. 1280