to change a decimal to a fraction, we simply "use as many zeros as there are decimals and lose the dot".
say in this case, 0.8 has 1 decimal, so we'll use one zero at the bottom,
[tex]\bf 0.\underline{8}\implies \cfrac{08}{1\underline{0}}\implies \cfrac{8}{10}\qquad \stackrel{simplified}{\implies }\qquad \cfrac{4}{5}[/tex]
