In a bag $$3$$ balls are white
$$2$$ balls are red
$$5$$ balls are black
Total number of balls $$=3+2+5=10$$
Number of possible outcome $$=10$$
Number of favourable outcome $$=3+2=5$$
We know that
$$P(E)=\dfrac{Number\ of\ favaourable\ outcome}{Number\ of\ all\ possible\ outcome}=\dfrac{5}{10}=\dfrac{1}{2}$$