Answer :

[tex]0.04= \frac{4}{100}\to\frac{1}{25} \\\\ \frac{1}{10} \ of \ x= \frac{1}{25} \\\\ \frac{1}{10}*x=\frac{1}{25} \\\\ \frac{x}{10}=\frac{1}{25} \\\\ \boxed{x=\frac{10}{25}\to\frac{2}{5}}[/tex]