In a box, there are 3 red marbles, 3 blue marbles and 7 green marbles. If 2 marbles are picked randomly, find the probability of picking two non-green marbles.
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When picking without replacement, use combinations: favourable $\binom{\text{wanted}}{2}$ over total $\binom{\text{all}}{2}$.
The number of non-green marbles is $3+3=6$; the total number of marbles is $3+3+7=13$. The required probability is $\dfrac{\binom{6}{2}}{\binom{13}{2}}=\dfrac{15}{78}=\dfrac{5}{26}$. \(\Rightarrow\) Both chosen marbles are from the 6 non-green marbles.