A sum of money becomes four times itself in 15 years at simple interest. What is the rate of interest per annum?

Let $P$ be the principal amount.
Let $A$ be the amount after time $T$.
Let $R$ be the rate of simple interest per annum.
Let $T$ be the time in years.

The formula for simple interest ($SI$) is:

$$SI = \frac{P \times R \times T}{100}$$

The total amount ($A$) is given by:

$$A = P + SI$$

From the problem statement:
The sum of money becomes four times, so $A = 4P$.
The simple interest rate $R = 5\%$ per annum.