
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
A certain institution has 840 workers, and they are using systematic sampling to conduct a questionnaire survey on 42 people. The workers were numbered from 1 to 840, and the interval number k was found to be $k=\frac{840}{42}=20$, which means one person is chosen out of every 20. Given that number 21 was selected, find the number of the selected 42 people whose numbers fall within the interval [421, 720].

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