
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
An environmental protection department conducted a field measurement of the environmental condition at a certain location. According to the measurements, the pollution index at this location is equal to the ratio of the pollution intensity of nearby pollution sources to the distance from these sources. It is known that two chemical plants (pollution sources), A and B, which are 30km apart, have pollution intensities of 1 and 4, respectively. The pollution index at any point on the line connecting the two chemical plants is equal to the sum of the pollution indices from both chemical plants at that point. Now, it is planned to build a park on the line between them to minimize the pollution index from both chemical plants. The park should be built ______ km away from chemical plant A.

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