Answer: 7/12
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Explanation:
There are B = 3+5+4 = 12 marbles total, of which A = 12-5 = 7 are not black. Or you could note that we have A = 3 red + 4 white = 7 marbles that are not black.
The probability of selecting a non-black marble is A/B = 7/12
7/12 = 0.5833 = 58.33% approximately
