Answer
$\dfrac{7}{30}$
Work Step by Step
We know that probability$=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}.$
Since there are $7$ black marbles and $5 + 10 + 8 + 7 = 30$ marbles in total, then
$P(\text{black marble})=\dfrac{7}{30}$
