At what rate of simple interest can a sum of money become five times of itself in 16 years?

Let the Principal amount be $P$.The amount becomes 5 times of itself, so Amount ($A$) = $5P$.Simple Interest ($\text{SI}$) = $A-P=5P-P=4P$ Time ($T$) = 16 yearsFormula:
$$\text{SI}=\frac{P\times R\times T}{100}$$
$$4P=\frac{P\times R\times 16}{100}$$
$$4=\frac{16R}{100}$$
$$R=\frac{4\times 100}{16}=\frac{400}{16}=25\%$$