A sum of Rs 2000 will become Rs 2400 in 12 months at some rate of simple interest. Find the rate of interest per annum.

40%

The formula for Simple Interest = $\frac{P \times R \times T}{100}$

let rate of interest per annum = r%

Simple Interest = Amount - Principal = 2400 - 2000 = 400

Thus,              400   = $\frac{2000 \times r \times 1}{100}$

              ⇒   2000r   = 400 x 100

               ⇒       r     =  40

Thus, the rate of interest per annum is 40%