Reena took a loan of Rs. 1200 with simple interest for as many years as the rate of interest. If she paid Rs. 432 as interest at the end of the loan period, what was the rate of interest? 

Suppose that the rate of interest is $x$

The simple interst for the amount $P$ in $x$ years with rate of interst $x$ is $\frac{P{x}^2}{100}$

Given that 

$\begin{array}{rcl}\frac{P{x}^2}{100}& =& 432\\ \frac{1200x^2}{100}& =& 432\\ x^2& =& \frac{432}{12}\\ & =& 36\\ x& =& 6\end{array}$

Therefore, the rate of interst is$6\%$