A sum triples itself in 9 years at simple interest. Find the rate of interest per annum.

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**Solution:**

1. Let the principal amount be $P$.

2. Since the sum triples itself in 9 years, the amount after 9 years is:
   $$A = 3P$$

3. Simple Interest (SI) is the difference between amount and principal:
   $$SI = A - P = 3P - P = 2P$$

4. The formula for simple interest is:
   $$SI = \frac{P 	imes R 	imes T}{100}$$
   where $R$ is the rate of interest per annum and $T$ is time in years.

5. Substitute the known values:
   $$2P = \frac{P 	imes R 	imes 9}{100}$$

6. Divide both sides by $P$ (assuming $P 
eq 0$):
   $$2 = \frac{9R}{100}$$

7. Multiply both sides by 100:
   $$200 = 9R$$

8. Divide both sides by 9 to find $R$:
   $$R = \frac{200}{9} = 22 \frac{2}{9} \%$$

**Answer:** The rate of interest per annum is $22 \frac{2}{9} \%$.

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