This means that if I borrow 100 rupees at 3% per annum, I have to pay an interest of 3 rupees in a year.

How much did I borrow at 3% p.a. to pay 135 rupees as interest?

Here are the steps to solve these kind of problems:

  1. Identify the unknown. In this case, let X be the amount borrowed
  2. Write the equation. In this case, \frac{\left(X\cdot3\right)}{100}=135
  3. Solve for the unknown