Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$0.5$
We know that probability of the event E is $P(E)=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}=\frac{n(E)}{n(S)}$. We have $E=\{2,4,6,8,10\}$, so $n(E)=5$ and $n(S)=10$. Thus, $P(E)=\dfrac{5}{10}=0.5$.