Dongdong’s family of 4 went to McDonald’s for lunch. It costs 12 yuan to buy a single portion, and buy 3 portions to get 1 portion free. If you buy 3 portions at a time, how much is each portion cheaper?
东东一家4人到麦当劳吃午餐，单独买一份需要12元，买3份送1份，如果1次买3份，每份便宜多少元？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">12 - 9</gadget>
<output>3</output>

<result>3</result>