A sum of Rs 15000 is deposited in two banks. Bank A pays 7% and Bank B pays 9% simple interest per year. After one year the interest from Bank B is Rs 390 more than the interest from Bank A. How much (in Rs) was deposited in Bank A?

Let Rs $x$ go to Bank A and Rs $y$ to Bank B. $x+y=15000$ and $0.09y-0.07x=390$, that is $9y-7x=39000$. Substituting $y=15000-x:135000-9x-7x=39000$, so $16x=96000$ and $x=6000$, giving $y=9000$. Check: 6000 + 9000 = 15000, and 810 - 420 = 390. 9000 is the Bank B deposit, the swapped unknown; 7500 splits the sum equally, which would give interests of 525 and 675 differing by only Rs 150; 420 is the interest Bank A pays, not the amount deposited there.