$a)0,8=\frac{8}{10}=\frac{4}{5}$

$b)3,5=3\frac{5}{10}=3\frac{1}{2}$

$c)0,18=\frac{18}{100}=\frac{9}{50}$

$d)6,6=6\frac{6}{10}=6\frac{3}{5}$

$e)4,25=4\frac{25}{100}=4\frac{1}{4}$

$f)5,75=5\frac{75}{100}=5\frac{3}{4}$