The simple interest received on a sum is 81/100 of the sum. The number of years is equal to the annual rate of interest. What is the annual rate of interest?

Simple interest (SI) is $$\frac{81}{100}$$​ of the principal (P).

The number of years t is equal to the annual rate of interest r.

The formula for simple interest is:

$$SI = \frac{P \times r \times t}{100}\\ \ \\ \bold{Solution}:\\ \ \\ \frac{81}{100}\times P = \frac{P \times r \times r}{100} \\ \ \\ \frac{81}{100} = \frac{r^2}{100} \\ \ \\ r^2= 81\\ \ \\ r =\sqrt{81}\\ \ \\ r= 9 \%$$  

Thus, correct option is (c).