Answer
$\dfrac{3}{5}=0.6$
Work Step by Step
Use the formula for a probability $P$ as: $\text{Probability}=\dfrac{\text{Number of favourable outcomes}}{\text{Total number of possible outcomes}}=\dfrac{n(E)}{n(S)}$
Here we have:
Number of white balls = $9$;
Number of orange balls = $3$.
Number of green balls = $8$.
So, the number of white or orange ball is $9+3=12$ , this means that $n(E)=12$ .
There are 20 balls in total, so $n(S)=20$.
Thus, the probability is:
$P\left(\text{selecting a white or orange ball}\right)=\dfrac{12}{20}=\dfrac{3}{5}=0.6$
