a)

$0,5=\frac{5}{10}=\frac{1}{2}$  

$0,13=\frac{13}{100}$

$0,09=\frac{9}{100}$

$0,121=\frac{121}{1000}$

$0,505=\frac{5050}{1000}=\frac{101}{200}$