
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
People enter the subway uniformly from the street. After passing through the turnstiles, they end up in a small hall before the escalators. The entrance doors have just opened, and initially, the hall before the escalators was empty, with only one escalator running to go down. One escalator couldn't handle the crowd, so after 6 minutes, the hall was halfway full. Then a second escalator was turned on for going down, but the crowd continued to grow – after another 15 minutes, the hall was full.

How long will it take to empty the hall if a third escalator is turned on?

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