There are $3$ odd numbers out of $6$, so the probability is $\frac36=\frac12$. 
A fair $6$-sided die is rolled once. What is the probability that the number on the top is an odd number? (adapted from 2011 AMC 8 Problem, Question \#18) 
$\frac16$ 
$\frac13$ 
$\frac12$ 
$\frac23$ 
$\frac56$ 