b) 12,5x = 25%;

b) 12,5.x = 25%

$\begin{array}{l}\frac{125}{10}\mathrm{x}=\frac{25}{100}\\ \frac{25}{2}\mathrm{x}=\frac{1}{4}\\ \mathrm{x}=\frac{1}{4}:\frac{25}{2}\\ \mathrm{x}=\frac{1}{4}.\frac{2}{25}\\ \mathrm{x}=\frac{1}{50}\\ \mathrm{Vậy} \mathrm{x}=\frac{1}{50}\end{array}$