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 outcomes of one red ball $$=10$$
and number of favourable outcome of one red ball $$=2$$
We know that
$$P(E)=\dfrac{Number\ of\ favourable\ outcome}{Number\ of\ all\ possible\ outcome}=\dfrac{2}{10}=\dfrac{1}{5}$$