#### Answer

$\dfrac{17}{20}\text{ or }0.85$

#### Work Step by Step

Use the formula for a probability $P$ as: $\text{Probabilit
y}=\dfrac{\text{Number of favourable outcomes}}{\text{Total number of outcomes}}=\dfrac{n(E)}{n(S)}$
Here we have:
Number of orange balls = $8$
Number of white balls = $9$
Number of green balls = $8$
Total number of balls = $20$
So, the number of white or green marbles is $9+8=17$.
This means that:
$n(E)=17$ and $n(S)=9+8+3=20$
Thus, the probability is:
$P\left(\text{selecting a white or green marble}\right)=\dfrac{17}{20}=0.85$