
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
There are some red, white, and black balls of the same size in a bag. If one ball is drawn from the bag, the probability of drawing a red ball is 0.42, and the probability of drawing a white ball is 0.28. If there are 21 red balls, then there are $\boxed{\text{black balls}}$.

Present the answer in LaTex format: \boxed{Your answer}
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