yuan per bottle of milk, buy 3 bottles and get 1 bottle free. I bought 3 bottles at a time, how much is each bottle cheaper?
每瓶牛奶8元，买3瓶送1瓶．一次买了3瓶，每瓶便宜多少元？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">8 / 4</gadget>
<output>2</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<result>2</result>