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The question is the same as drawing a Black marble. That's one way to look at it. Another is to draw a gray and then subtract that probability of not drawing a gray. I'll do both. Choose the one that is clearest in your mind.
Method One
Total number of marbles = 21 + 11 = 32 marbles of both colors. Probability of drawing a black marble =
P(B) = P(~gray) = 11 / 32
Method Two
A. Find the probability of drawing a gray marble.
There are 21 gray marbles and 32 total
P(G) = Number of Gray's / total
P(G) = 21 / 32
P(~G) = 1 - 21/32
P(~G) = 32/32 - 21/32
P(~G) = 11/32 <<<< Answer
