Chapter 13 - Counting and Probability - 13.3 Probability - 13.3 Asses Your Understanding - Page 866: 57

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

We know that $\text{probability}=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}=\frac{n(E)}{n(S)}.$ Hence we have: $9+8=17$ white or green marbles so $n(E)=17$ $n(S)=9+8+3=20$ marbles Therefore, $P\left(\text{selecting a white or green marble}\right)=\dfrac{17}{20}=0.85$