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

Let the principal amount be $P$.
Since the amount triples over $25$ years, the final total amount ($A$) is $3P$.

$$\text{Simple Interest (S.I.)}=A-P=3P-P=2P$$

The formula for Simple Interest is:

$$\text{S.I.}=\frac{P\times R\times T}{100}$$

Given time $T=25$ years:

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

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

$$2=\frac{R}{4}⟹R=8\%$$

Hence, the rate of interest is $8\%$ per annum.