
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 vendor bought a batch of pens and sold them. Some customers bought one pen for 10 rubles, and some bought 3 pens for 20 rubles. It turned out that the vendor received the same profit from each purchase. Find the price at which the vendor bought the pens.

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