An amount will be triple itself in 15 years at a certain rate of simple interest per annum. The rate of interest per annum is:

Let the principal amount be P.
Since the amount triples itself in 15 years, the total Amount (A) becomes 3P.
Time (T) = 15 years

The Simple Interest (SI) earned is:

SI=A−P=3P−P=2P
The formula for simple interest is:

SI=
100
P×R×T
​

Substituting the known values into the formula:

2P=
100
P×R×15
​

Dividing both sides by P:

2=
100
15R
​

R=
15
200
​
=
3
40
​
=13
3
1
​
%