3. (7 points) A seller bought a batch of pens and sold them. Some customers bought one pen for 10 rubles, while others bought 3 pens for 20 rubles. It turned out that the seller made the same profit from each sale. Find the price at which the seller bought the pens.
Answer: 5 rubles.

Solution. Let the purchase price of a pen be $x$. Then the profit from one pen is $10-x$, and from 3 pens is $20-3x$. Solving the equation $10-x=20-3x$, we get $x=5$.

Criteria. Correct solution by any method: 7 points.

If it is not justified that the purchase price of the pen should be 5 rubles, but it is verified that the condition is met in this case: 4 points.

Only the correct answer without any explanation: 2 points.