
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 were 6 boxes in a store, each with a mass of 15, 16, 18, 19, 20, and 31 kilograms. Two companies bought five boxes, with one company taking twice the mass of apples than the other. Which box remained in the store?

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