An amount will triple itself in $25$ years on simple interest. Find the rate of interest.

Let the principal amount be $P$.
Since the amount triples itself in $25$ years, the final amount $A=3P$.

The Simple Interest ($SI$) earned is:

$$SI=A-P=3P-P=2P$$

Using the Simple Interest formula:

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

Substitute the known values ($SI=2P$ and $T=25$):

$$2P=\frac{P\times R\times 25}{100}$$

$$2=\frac{R\times 25}{100}$$

$$2=\frac{R}{4}$$

$$R=2\times 4=8\%$$

Thus, the rate of interest is $8\%$.