To find the probability of selecting a white marble from the bag, we first need to determine the total number of marbles.
The numbers of each color of marble are:
- Red: 6
- Green: 8
- Black: 4
- White: 12
Now, we can calculate the total number of marbles:
\[ \text{Total marbles} = \text{Red} + \text{Green} + \text{Black} + \text{White} = 6 + 8 + 4 + 12 \]
\[ \text{Total marbles} = 30 \]
Next, we find the number of white marbles, which is 12.
The probability \( P \) of selecting a white marble is given by the ratio of the number of white marbles to the total number of marbles:
\[ P(\text{white}) = \frac{\text{Number of white marbles}}{\text{Total number of marbles}} = \frac{12}{30} \]
To simplify this fraction, we can divide both the numerator and the denominator by 6:
\[ P(\text{white}) = \frac{2}{5} \]
Thus, the probability of selecting a white marble is
\[ \boxed{\frac{2}{5}} \]