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

$\dfrac{17}{20}\text{ or }0.85$
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: $P\left(\text{selecting a white or green marble}\right)=\dfrac{17}{20}=0.85$