A sum becomes 4 times at simple interest in 10 years. What is the rate of interest?

$$\eqalign{ & {1^{{\text{st}}}}\,{\text{Method}}: \cr & {\text{Let rate is }}R\% \cr & {\text{Now}}, \cr & P = 100, \cr & A = 400, \cr & I = 400 - 100 = 300, \cr & {\text{Time}},\,T = 10\,{\text{years}} \cr & I = \frac{{PTR}}{{100}} \cr & {\text{Or}},R = \frac{{ {100 \times I} }}{{PT}} \cr & {\text{Or}},R = \frac{{ {100 \times 300} }}{{ {100 \times 10} }} \cr & {\text{Hence}},{\kern 1pt} R = 30\% \cr} $$

2^nd Method :
Here, the sum become 4 times that means 100, become 400.
Rate of such question is given by
$$R = \frac{{{\text{interest}}}}{{{\text{time}}}} = \frac{{300}}{{10}} = 30\% $$

3^rd Method :
Here, 300% of rise in the sum so
$$\eqalign{ & 100 - - - 300\% \uparrow - - - {\kern 1pt} 400; \cr & R = {\frac{{{\text{total}}\,{\text{percentage rise}}}}{{{\text{given time}}}}} \cr & \,\,\,\,\,\,\,\, = \frac{{300\% }}{{10}} \cr & \,\,\,\,\,\,\,\, = 30\% \cr} $$