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