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

Answer: Option C

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

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
$$\begin{array}{rl}& 100---300\%\uparrow ---400;\\ & R=\frac{\text{total}\text{percentage rise}}{\text{given time}}\\ & =\frac{300\%}{10}\\ & =30\%\end{array}$$