
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 fast train and a slow train are moving towards each other. The length of the fast train is 315 meters, and the length of the slow train is 300 meters. A person sitting on the slow train observes that the fast train passes by in 21 seconds. How long does it take for a person sitting on the fast train to see the slow train pass by in seconds?

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