Answer
$\dfrac{1}{6}$
Work Step by Step
We know that $\text{probability}=\dfrac{\text{number of favorable outcomes}}{\text{number of all outcomes}}=\dfrac{n(F)}{n(S)}.$
There are $5$ white marbles so $n(F)=5$
The total number of possible outcomes is $n(S)=5+10+8+7=30$.
Therefore,
$\text{probability of getting a white marble}=\dfrac{5}{30}=\dfrac{1}{6}.$
